Start with the right checks.
Read only means you can view the workbook but cannot save changes to that original in the normal way. Several different causes can produce the same symptom.
What to do next.
Read the message above the worksheet. Note whether it mentions Protected View, permissions, activation or another person editing.
Confirm that you opened the intended file using the correct work account.
For a shared file, ask the owner to confirm editing access. Do not bypass protection or disable antivirus to force a change.
If a separate copy is appropriate, use Save As with a different name in an approved location. Check cloud storage capacity if saving is blocked.
Check the result.
Make a harmless test change in an authorized copy, save it, close it and reopen it. Confirm it is the file you intend others to use.
Stop and get help if…
The workbook contains protected business data, has an unknown password, or the owner has not approved editing.