A question about software updating

Start by visiting this site: https://support.apple.com/en-us/HT211683

Check that your Mac is eligible and able to run Big Sur.

Then use the link on that page to download and run the installer to upgrade.

Also check your applications, some older versions (e.g. Adobe, Microsoft Office, etc.) will run under High Sierra but will require new versions for Big Sur.

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

macOS High Sierra - Technical Specifications

macOS Mojave - Technical Specifications The Age of this computer will Govern what newer version of macOS it will Qualify to install.

If this computer does Not Qualify for anything newer than High Sierra NO - there is no way to get a newer version of macOS installed on this computer/

The Only Option is to purchase a newer or new Apple Computer that either Qualifies to run a newer version of macOS or a New Computer that already has the Newest Version of macOS - Monterey macOS 12