[Swlug] checking accuracy / authenticity of Mint Mate ISO 19.1

[bill]

Hi team,

I currently run Mate 17.3 Rosa on my laptop. I've downloaded the Mate 
19.1 64-bit ISO, and I have the files sha256sum.txt and sha256sum.txt.gpg

How do I now use these files to check the accuracy and authenticity of 
the newly downloaded iso (downloaded from Bytemark yesterday)?

Cheers,

Bill :)