Im trying to update but it keeps saying that there is not enough space on disk how do I get more space?

• See used and available storage space on your Mac especially this about " Other "Other: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.

What is “Other” storage on a Mac, and how can I clean it out?

Regarding buying extra memory - RAM might be upgradable but you will have to verify this computer can be upgraded to more memory as often times the Memory and Internal Drive are soldered to the LogicBoard and can not be upgraded.