How do I upgrade my Mac from macOS 10.3 to 11.0?

Back up your data, check your applications for compatibility, click here, and if your Mac model is recent enough, then on the download link for Big Sur or newer. If you actually meant 10.3 instead of 10.13, it isn't.

Catalina and higher won't run 32-bit software; download and use Go64 as well as using the first link above.

Alternatively, use a product such as LibreOffice instead.

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Another vote for  the Free LibreOffice, it handles MS Office files like Word & Excel, & Page Layout…

https://www.libreoffice.org/