how do I upgrade MacBook from OS 10.12.6 to a version like 10.15 that supports editing Excel spreadsheet

What macOS you can update to will be dependent on the model / year of Mac you have. Look in the Apple Menu->About this Mac to get the specific model / year of your Mac.

Then you can see what macOS version you can install: link-> How to get old versions of macOS - Apple Support

If you have a more recent Mac that supports it you can follow the steps here to install macOS 12 Monterey: link-> How to upgrade to macOS Monterey - Apple Support

See this: Upgrade to macOS Monterey - Official Apple Support

How to get old versions of macOS - Apple Support

Or...

Look into the Free LibreOffice, it handles MS Office files like Word & Excel, & Page Layout…

https://www.libreoffice.org/