Common knowledge and standard game mechanics.
You dbl click the scroll binder, target the 115 scroll and it gets added to the binder. You have to add like to like, you can't make a 120 magery out of 10 random crap 115s but you can make a 120 mage out of 10 115 magery scrolls.
When youadd the tenth scroll, the binder turns into a 120. You can also turn 10's into 15's, it takes 12 10's to get 1 15.