How to unprotect Excel sheet if forgot the password

You don't need to change the file extension to zip or arj or whatever.

You can just right click the xlsx file and choose to open it with a ZIP editor.
I use and recommend 7-zip.

Thereafter, the procedure is the same as explained by EdwardGreen and ToddSumrall, i.e.:

1. Find the directory called \xl\worksheets\ and spot the (first) XML file related to the sheet you want to unlock.
2. Right-click the XML file and choose EDIT -> your text editor will open the XML file.
3. Inside the XML file, find the "sheetProtection" tag and entirely delete it (i.e. from the "<" symbol just before "sheetProtection", to the nextcoming ">" symbol, both included).
4. Save the XML file
5. Close the XML file and when prompted, confirm you want to reload the modified xml file into the xlsx archive.