How do we amend VAT return submitted on line last month
On the next return if...
If the error is less than the prescribed limits (headline figure at least £10,000 but there's more detail to it than that) then the correction has to be made on the next return.
If above the prescribed limit then form VAT652 has to be used.
If VAT has been underpaid then specific disclosure to HMRC is required to mitigate penalties.