I have done a CIS return but missed a transation. it now wont let me change it as it has already been filed how do i correct this

Hello Bridgewood77, 

Welcome to the Community page, 

You will need to ring the helpline on 0808 234 5337 and the agent will be able to set up a screenshare with you and get all the information they need so they can raise an unfile request for you, so that you can then change the information you need and resend it again.