Quick answer
Correct the record in your software and the fix goes into your next quarterly update. Updates are cumulative from the start of the tax year, so each one replaces the last and you don’t need to resend earlier updates. After the final declaration, you amend it the same way you’d amend a tax return.
Why you don’t resend old updates
Each quarterly update covers the whole year so far. Your second update includes periods one and two, your third includes one to three, and so on. Fixing a figure from an earlier quarter just means it’s correct in the next update you send.
Mistakes in the last quarter
Errors found after your fourth update can be corrected when you prepare your final declaration.
After the final declaration
You can amend it in the same way as a Self Assessment return, usually within 12 months of the 31 January deadline. See HMRC’s guidance on correcting a tax return.
Avoiding mistakes
Reconcile your bank account in your software every month.
Keep business and personal spending separate.
Review uncategorised transactions before each update.