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First, keep copies of your backup archive locally. My rule is that there should always be two local copies: one on the computer you will be using, one on external media (USB flash drive, NAS, a service like Dropbox or OneDrive, etc). The idea is that if you mess something up by accident you will have at least one copy of the backup archive to go back to.

After you've done that, take a deep breath because we're about to get dangerous. We will have to delete a few folders from your site: administrator, api, cache, cli, components, includes, language, layouts, libraries, modules, plugins, templates, tmp (some folders may not exist; that's okay). This is important; you can't restore a Joomla 4 site on top of a Joomla 5 site, it will cause a mix of files and the restored site will be unusable.

Then, upload the backup archive file(s) and kickstart.php (Akeeba Kickstart Core) to your site and visit Kickstart's page (/kickstart.php on your site) to start the restoration.

IMPORTANT! Some backup archives may consist of multiple files with extensions .jpa, .j01, .j02, … or .zip, .z01, .z02, … Remember to download and upload all of those files. They are not separate archives, they are parts of the same archive. Think of it like War & Peace. Too big to print in one bound volume, it's usually printed in two volumes. You can't read one and make sense of the whole. You need to read both volumes to say you have read the entire book. The same idea applies with those files. Each file is a volume of the archive (whole book).