Thanks for the replies,  I know quite a bit about Electrum private key, public keys, spending and storage in air gapped or hardware wallets etc,  I just need to know how to trust a public key in Electrum on Linux Mint v22

I copied these 2 files to the Downloads folder of Linux Mint v22
electrum-4.5.8-x86_64.AppImage
electrum-4.5.8-x86_64.AppImage.asc

Then entered these commands in the terminal

gpg --lsign-key 6694D8DE7BE8EE5631BED9502BD5824B7F9470E6

gpg --verify ~/Downloads/electrum-4.5.8-x86_64.AppImage.asc

gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: 6694 D8DE 7BE8 EE56 31BE  D950 2BD5 824B 7F94 70E6

So I have trusted the ID in "Passwords and Keys" and in the terminal but still have the warning of not certified with a trusted signature.   
.