Hi JLC and welcome to The Forum!
Midland County is a very busy place these days. It and about another dozen or more Counties are in The Permian and the Delaware Basins (geological terms) and there are more Drilling Rigs in that area now than anywhere else in the world. Your mineral interests could be quite valuable.
There are literally hundreds of companies and individuals that are trying to buy minerals and royalties in West Texas and Southeastern New Mexico. You will be receiving offers for years to come.
What any of us will need to help you is a copy of your lease to review and copies of the last 3 check stubs you have received in order to give you a ballpark figure for you to begin with.
To answer one of your questions, it is highly doubtful that you will be able to alter or amend your relative's lease.
Yes, most of the websites you look at want you to submit your information so they can make an offer. A website like this one or perhaps NARO's website (National Association of Royalty Owners) are the type you will get the most free advice from.
Hope this helps -
Charles Emery Tooke III