How to give highest trust level to an OpenPGP certificate in Kleopatra?

I'd like to give the highest trust level to a certificate. I need to do so, as this certificate is mine but I don't have exported it before formatting my PC.

I have downloaded it from a public key server and I have my private key saved in KeePass, but I don't know what to do next.

Solution

I'm sorry I cannot tell you how to do it in Kleopatra, but it is rather easy on the command line.

If you haven't imported the private key yet, save it to a file (let's say, key.pgp, name and extension don't matter). Now run gpg --import <key.pgp. Your key should be listed using gpg -K now.
For setting trust (the highest trust level in OpenPGP is "ultimate", which is usually only applied to your own certificates, as you want to do in this case), run gpg --edit-key [key-id] (printed in gpg -K above). In the edit key menu, run trust; and enter 5 (followed by Return for "I trust ultimately". Confirm with y (again followed by Return), and finally run save to store the modified key and quit GnuPG.