To amend the SA100 you can click on this link. Alternatively you can also follow the navigation as shown below.

Navigation: Self Assessment >> Client Specific >> Action >> Edit >> Edit Return Details >> Amended >> Update

After you have navigated your way to the client specific SA100 dashboard, select the 'Edit' from the 'Action' drop down.

Select 'Edit Return Detail'.

Once you select 'Edit Return Detail', a 'Edit Return Detail' prompt box will appear.

Select 'Amended'. 

Select 'Update' to complete the amendment.