If you’ve run the Year End Tool, then you will have to undo the 2 steps it takes to reverse this process. That is - delete the journal, and release the lock.
You can roll back a VAT return by viewing it and clicking the “Roll back return” button. However, it’s worth noting that this only affects your QuickFile entries. If you need to make changes to the ones that have been submitted to HMRC, you would need to contact them.