You need to flash a custom recovery (TWRP) using a PC tool called Odin. Find a stable LineageOS build for the on the XDA Developers forum.

Back up all essential data. This process wipes all data. Install TWRP Recovery:

on how to flash a custom Android 10 ROM, or would you like to see newer tablet alternatives that natively support modern software?

have successfully ported LineageOS 17.1 (which is based on Android 10) to various Galaxy Tab A models.