HI Chupi,
We make it hard for you to delete bills when you’ve already paid tax on them, because it can get tricky.
What you need to do is the following.
1) Find the VAT period that the bill was in, you see this on Tax > Overview click on the Paid Tab then click on Vat items for this period at the bottom, you’ll see your bill listed there.
2) While in the correct VAT period, mark the VAT period as unpaid (you’ll see the link on the right).
3) Then go and delete your Bill.
4) Then go and mark that VAT period as paid, and Bullet will magically fix your next VAT return.
Hope that helps.
Pete