i tried upgrading from yosimite to high sierra and now i don’t have enough free space to download macOS or go back to the one i used to have which is Yosimite

A backup prior to an upgrade is always a good idea.

Connect an external disk and then implement Time Machine for a backup.

Time Machine is part of macOS. All that is necessary to backup is an external disk drive that you can configure to receive the backup file from Time Machine.