A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
All the above answers ASSUME that the protection can be removed or bypassed.
But what if your workbook is protected by someone else and you don't have the password to remove the protection or permissions to edit macros?
For this, a simple and easy solution is to copy the unprotected data or entire sheet into a new blank Excel workbook or new unprotected sheet (if it's just one column, you could even use Notepad). Then use the Find/Replace function in the new workbook.
Once you have completed all the Find/Replace you need to do, copy the modified data and paste back into the unprotected range in the protected sheet or workbook.
I agree that it's ridiculous that Excel does not allow you to find/replace in unprotected cells even if the sheet is protected.