This is how to do it:
1. Sign in to your Analytics account as an administrator at https://www.google.com/analytics/web/.
2. Click the admin link at the top right of any Analytics page.
3. Click the account from which you want to unlink your AdWords account.
4. Click the Data Sources tab.
5. Click the AdWords tab.
6. Click Unlink for the AdWords account.
7. Easy! Your AdWords account is now unlinked from you Analytics account!
In the case where you need to link an Adwords account to google Analytics:
1. Sign in to your AdWords account at https://adwords.google.com.
2. Click the Tools and Analysis tab, then click Google Analytics.
3. Click the admin link at the top right of any Analytics page.
4. Click the account to which you want to link the AdWords account.
5. Click the Data Sources tab.
6. Click the AdWords tab.
7. Click Link Accounts.
8. Select the Analytics profiles in which you want the AdWords data to be available.
Make sure that on you adwords settings you have selected the Auto-tagging. To do that, click on MyAccount and click on Edit where you see Auto-tagging. Switch to yes.