1. Go to http://ask.fm/account/settings/services and click on "Remove connection" under the Twitter service.
If the "Remove connection" button does not appear, proceed to "Profile" settings and click on the "Change password" button in the bottom of the page which will enable you to create a password for the said account, which will make the "Remove connection" show up in the "Services" tab.
2. Log out from this account, and log in again into the account you want to connect your Twitter to.
3. Again, go to http://ask.fm/account/settings/services and click on "Connect" under the Twitter service.