Open the file and save it as Open Document Spreadsheet (.ods).
File>Save as… File Type: “Open Document Spreadsheet (.ods)”.
You are using Windows, right?
Open the file explorer.
View>Options>Folder Options…
Tab “View”
“Hide extensions of known file types” = OFF
By default this is on. This is the worst default since Windows 95 since it hides away the most important info about a file.
Now you see the full name of your spreadsheet, say “MySpreadsheet.ods” with .ods extension. Rename the file to “MySpreadsheet.zip” (which is difficult to do with default settings). Ignore any stupid warning. Every byte of the file remains intact. We just change the label.
Open the zip file.
Load content.xml into a text editor. You may have to extract a copy out of the zip archive.
Replace table:protected="true"
with table:protected="false"
Save content.xml and push it back into the zip.
Rename the document file back to “MySpreadsheet.ods”.
If anything went wrong, you still have the xlsx.