Part 1 of 2
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.
Safari will open a redirect and open the Apple Store where the download can commence.
Does this computer appear as Listed and qualifying for an upGrade to a newer version of macOS?
The below links will help identify which version of macOS this computer may Qualify to run.
Computer Qualifying for macOS 13 Ventura
Computer Qualify for the Monterey upgrade
macOS Big Sur 11 hardware requirements
Make Time Machine Backup is very useful
Part 2 of 2
If the user whats the latest and greatest versions of macOS, all is not lost.
Explore the Apple Trade-In Process where the Value of the exiting device could be used against the cost of a new Apple Computer, iPhone or iPad
Or Shop Refurbished Apple computers at an Apple Reduced Cost
Have used both previously and have been satisfied each time.