I need an older than 13.0 version of LightRoom Classic to update an old catalog from 2018.

If your computer cannot support LrC v13, or 14. If you need to use LrC v12, then you will need to contact Adobe to request a link to a download of the older version (this involves a 2 major version rule, i.e now that v14 is out, the Adobe Creative Cloud Install show v14, and v13, but not v12)

You can start a chat to contact them.

Lightroom-Classic v14 can 'open' (by upgrading) catalogs from previous versions.

What have you tried that you "cannot find any way...."? Any error messages?

What Version of Lightroom-Classic created the "2018" catalog?

I have seen a couple of forum posts that v5 and v6 catalogs cannot be upgraded.

The current LrC 14.0.1 will upgrade the catalog from 2018.