You can now tell HMRC about an overclaimed amount as part of your next claim. You will be asked when making your claim whether you need to adjust the amount down to take account of a previous error. Your new claim amount will be reduced to reflect this. You should keep a record of this adjustment for 6 years.
If you have made an error that has resulted in an underclaimed amount, you should contact HMRC to amend your claim. As you are increasing the amount of your claim HMRC will need to conduct additional checks.