right-click the selection | Edit Selection ()
A dialog appears displaying the sequence. You can add, remove or change the text and click OK. The original selected part of the sequence is now replaced by the sequence entered in the dialog. This dialog also allows you to paste text into the sequence using Ctrl + V ( + V on Mac).
right-click the selection | Delete Selection ()
If you wish to only correct only one residue, this is possible by simply making the selection only cover one residue and then type the new residue. Another way to edit the sequence is by inserting a restriction site. See Insert restriction site before/after selection.