Reports → HMRC:
if you have any accounts listed in the table, click the “view” link at the right for each one, then the “disconnect” button on the next screen, as Beth mentioned above.
Again, on Reports → HMRC, use the big blue “connect” button, select “organisation”, then “continue”. You will be redirected to gov.uk where you need to log in to the correct Government Gateway account that has your VAT details and then follow the prompts, eventually you’ll get redirected back to QuickFile with the account connected and listed in the table.
Once you have the right account (and only the right account) connected, go to Reports → VAT Returns → VAT Settings and make sure all the right boxes are ticked - enable online filing, use MTD features, and select your tax account from the drop-down (there should only be one option).