Contacts display names

Hi All,

Is there a way to change the display name of a contact? I've received an email sent to me and one other person, however their name simply displays as 'J', which next to my name is almost invisible as it starts with 'J'. Can I change the display name on the desktop app to use the person's actual name? Which I do know, and it it would make it easier to spot emails from them in my inbox.

I have tried looking this up before but everything thing I've found doesn't seem to work so I thought I'd ask the Redditors who usually know how to do everything!

Thanks in advance.