diff --git a/openspec/changes/add-update-command/tasks.md b/openspec/changes/add-update-command/tasks.md
index 412fc5053..c539e03e2 100644
--- a/openspec/changes/add-update-command/tasks.md
+++ b/openspec/changes/add-update-command/tasks.md
@@ -1,20 +1,20 @@
 # Implementation Tasks
 
 ## 1. Update Command Implementation
-- [ ] 1.1 Create `src/core/update.ts` with `UpdateCommand` class
-- [ ] 1.2 Check if `openspec` directory exists (use `FileSystemUtils.directoryExists`)
-- [ ] 1.3 Write `readmeTemplate` to `openspec/README.md` using `FileSystemUtils.writeFile`
-- [ ] 1.4 Update `CLAUDE.md` using markers via `FileSystemUtils.updateFileWithMarkers` and `TemplateManager.getClaudeTemplate()`
-- [ ] 1.5 Display ASCII-safe success message: `Updated OpenSpec instructions`
+- [x] 1.1 Create `src/core/update.ts` with `UpdateCommand` class
+- [x] 1.2 Check if `openspec` directory exists (use `FileSystemUtils.directoryExists`)
+- [x] 1.3 Write `readmeTemplate` to `openspec/README.md` using `FileSystemUtils.writeFile`
+- [x] 1.4 Update `CLAUDE.md` using markers via `FileSystemUtils.updateFileWithMarkers` and `TemplateManager.getClaudeTemplate()`
+- [x] 1.5 Display ASCII-safe success message: `Updated OpenSpec instructions`
 
 ## 2. CLI Integration
-- [ ] 2.1 Register `update` command in `src/cli/index.ts`
-- [ ] 2.2 Add command description: `Update OpenSpec instruction files`
-- [ ] 2.3 Handle errors with `ora().fail(...)` and exit code 1 (missing `openspec` directory, file write errors)
+- [x] 2.1 Register `update` command in `src/cli/index.ts`
+- [x] 2.2 Add command description: `Update OpenSpec instruction files`
+- [x] 2.3 Handle errors with `ora().fail(...)` and exit code 1 (missing `openspec` directory, file write errors)
 
 ## 3. Testing
-- [ ] 3.1 Verify `openspec/README.md` is fully replaced with latest template
-- [ ] 3.2 Verify `CLAUDE.md` OpenSpec block updates without altering user content outside markers
-- [ ] 3.3 Verify idempotency (running twice yields identical files, no duplicate markers)
-- [ ] 3.4 Verify error when `openspec` directory is missing with friendly message
-- [ ] 3.5 Verify success message displays properly in ASCII-only terminals
\ No newline at end of file
+- [x] 3.1 Verify `openspec/README.md` is fully replaced with latest template
+- [x] 3.2 Verify `CLAUDE.md` OpenSpec block updates without altering user content outside markers
+- [x] 3.3 Verify idempotency (running twice yields identical files, no duplicate markers)
+- [x] 3.4 Verify error when `openspec` directory is missing with friendly message
+- [x] 3.5 Verify success message displays properly in ASCII-only terminals
\ No newline at end of file
diff --git a/src/cli/index.ts b/src/cli/index.ts
index d428f098b..8ae9417a0 100644
--- a/src/cli/index.ts
+++ b/src/cli/index.ts
@@ -3,6 +3,7 @@ import ora from 'ora';
 import path from 'path';
 import { promises as fs } from 'fs';
 import { InitCommand } from '../core/init.js';
+import { UpdateCommand } from '../core/update.js';
 
 const program = new Command();
 
@@ -44,4 +45,19 @@ program
     }
   });
 
+program
+  .command('update [path]')
+  .description('Update OpenSpec instruction files')
+  .action(async (targetPath = '.') => {
+    try {
+      const resolvedPath = path.resolve(targetPath);
+      const updateCommand = new UpdateCommand();
+      await updateCommand.execute(resolvedPath);
+    } catch (error) {
+      console.log(); // Empty line for spacing
+      ora().fail(`Error: ${(error as Error).message}`);
+      process.exit(1);
+    }
+  });
+
 program.parse();
\ No newline at end of file
diff --git a/src/core/update.ts b/src/core/update.ts
new file mode 100644
index 000000000..93465a10d
--- /dev/null
+++ b/src/core/update.ts
@@ -0,0 +1,35 @@
+import path from 'path';
+import { FileSystemUtils } from '../utils/file-system.js';
+import { TemplateManager } from './templates/index.js';
+import { OPENSPEC_DIR_NAME, OPENSPEC_MARKERS } from './config.js';
+import { readmeTemplate } from './templates/readme-template.js';
+
+export class UpdateCommand {
+  async execute(projectPath: string): Promise<void> {
+    const resolvedProjectPath = path.resolve(projectPath);
+    const openspecDirName = OPENSPEC_DIR_NAME;
+    const openspecPath = path.join(resolvedProjectPath, openspecDirName);
+
+    // 1. Check openspec directory exists
+    if (!await FileSystemUtils.directoryExists(openspecPath)) {
+      throw new Error(`No OpenSpec directory found. Run 'openspec init' first.`);
+    }
+
+    // 2. Update README.md (full replacement)
+    const readmePath = path.join(openspecPath, 'README.md');
+    await FileSystemUtils.writeFile(readmePath, readmeTemplate);
+
+    // 3. Update CLAUDE.md (marker-based)
+    const claudePath = path.join(resolvedProjectPath, 'CLAUDE.md');
+    const claudeContent = TemplateManager.getClaudeTemplate();
+    await FileSystemUtils.updateFileWithMarkers(
+      claudePath,
+      claudeContent,
+      OPENSPEC_MARKERS.start,
+      OPENSPEC_MARKERS.end
+    );
+
+    // 4. Success message (ASCII-safe)
+    console.log('Updated OpenSpec instructions');
+  }
+}