You can edit comments which you've entered into the comment section of an activated checklist instance. Note: you can only edit a comment you made.
To edit a comment you have entered:
Open up your activated checklist and go to the step with the comment you would like to edit
Click on the three vertical dots on your comment box and select Edit message from the drop-down
Enter updates and changes as required and then click on update to capture your edits.
Note: that when comments are edited, the word edited will appear next to the comment in the comments section and also in the checklist instance report.