There's no way to do this naturally. You can fake it, but rolling yiour die, adding 1, and setting the max equal to the max you can roll. For instance if you are rolling d6, with 1s becoming sixes, your die results can be 6, 2, 3, 4, 5, 6. That is exactly the same as rolling d6+1, max 6. You have the same outcomes of 2, 3, 4, 5, 6, 6. Limiting to the max in this would look like this {1d6+1,6+0d0}dh1 dh1 means drop the highest. So if the d6+1 gets a 7 you use the other other result, 6, otherwise you keep the d6+1 result. 6+0d0 is used because in a group roll like that { } you cant combine dice and numbers - you must use both dice, or both numbers. The +0d0 tricks roll20 into treating that as a dice roll.