You're not able to undo a VAT Return, or unreconcile VAT reconciled transactions. You can either restore the backup taken or delete the return and resubmit it.
If you need to correct a previously reconciled VAT Return, the only option is to restore a backup taken before you reconciled the VAT Return.
Restoring a backup doesn't reverse a submission sent to HMRC. For example, you can't restore a backup, correct your return, and submit it again.
You can only submit your VAT Return to HMRC online once for each period. So only restore a backup if you've reconciled your VAT Return in error.
If you choose to delete the reconciled VAT Return within the software, instead of restoring, you'll need to resubmit the VAT Return.
NOTE: If you restore a backup, you'll need to repost anything since the last backup was taken.