How do i download the latest MacOS if it says i don't have enough space?

you can purchase an external HDD or SSD of sufficient capacity and move your larger files (videos, pictures, documents, etc...) to it. once they are on the EHD, you can simply erase the same files from your internal drive. and, if it's just going to be used for storage, you can use an HDD and save a few bucks over the cost of an SSD. FWIW, i personally keep very little on my internal drive.

also, you may want to read these two articles:

Optimize storage space on your Mac

How to free up storage space on your Mac

Backup and then temporarily move large data files (Photos and Music libraries and other large documents) and maybe also large user-installed applications to an external APFS-formatted drive. iOS backup files might also use much space so you might backup them to iCloud and temporarily delete backups on the internal disk. System "snapshots" should automatically free the space, if needed, but sometimes manually deleting them might need special tricks.

How much space do you have, and what versions are you updating from/to?

If you are low on space, not only an update fails, but your system performance degrades and ultimately your mac may cease to function properly.

It is usually recommended to have at least 10-15% free on any drive, and especially the system drive.

I am guessing that you may have a small internal drive, so as others have mentioned, you may need to clear some space by copying stuff off to an external. But don't just copy everything back and try to work with a full drive. It will not end well.