Before you start: find the Case IDs
A surveyor wrote roof areas in square feet, a choice was imported with the wrong code, a name is misspelled on twenty cases - Edit Submissions fixes answers across many cases from one spreadsheet, without opening each case. For a single case, use the pencil on the case page instead (Edit a submission).
Each row of the edit file points at one answer on one case, so you need the Case ID of every case you want to change. It is shown in the header of the case page (Case Id 17111789226954076B), in the Case ID column of a site's CASES list, and in any Data View or export of the form.
Edits are kept as a new version of the submission - the previous answers stay under VERSIONS on the case - but Zite does not undo an edit for you. Export the current values first if you may need them.