Dessie, it's all negotiable but they don't have to agree. You could strike the 15% and put in 20%, but with or without striking the 15%, it's customary to add a page, an addendum that says no matter what was said before, the royalty actually agreed to is 20%. The preceding is not legal language and I would suggest having someone with legal experience draft it.
As I said, they may not agree to it. The risk is that they walk away and you receive nothing.
In your situation I would demand payment before you send them an executed lease. If you don't have cash for a lawyer, you certainly don't have cash to sue them if they record your lease but not pay you, you could literally wind up worse off than you are now and I'd hate to see that happen....as it did to me at one time.