You must usually make any changes (‘amendments’) within 12 months of the filing deadline.
• use commercial software
• send a paper return or write to your company’s Corporation Tax office
You may be able to make changes to your Company Tax Return using HM Revenue and Customs (HMRC) online services.
Check if you’re eligible to use the service.
However, if this deadline has passed, and you believe that you have paid too much tax, there are some circumstances where you may be able to make claim for
Further guidance and information can be found here:
SACM12000 - Overpayment relief: Contents
SACM12150 - Overpayment relief: Form of claims
Please note, claims for ‘overpayment relief’ for Corporation Tax should be sent to:
Corporation Tax Services, HM Revenue and Customs, BX9 1AX
If the tax liability has increased, and you owe tax, you can make a voluntary disclosure.
Further guidance can be found here:
How to make a voluntary disclosure to HMRC