Step 1. Exit WoW
Step 2. Either click the drop down box above WoW and click uninstall or go into your control panel and uninstall it from there.
Step 3. Log into your Battle.net account.
Step 4. Select your currently subscribed WoW license/licenses.
Step 5. Click "Cancel Subscription".
Step 6. Log out of Battle.net
Step 7. There are no more steps you are done.