From this day forward, Elixirs of Mastery that are required to upgrade your proficiency with a certain School of Magic are no longer available. All you have to do now to increase Magic Mastery is select a tab with the School of Magic you wish to upgrade, click the plus icon to the right of the Mastery bar and pay the required sum.

Dragon Mastery is increased the same way, but please note that in order to upgrade your Dragon's Mastery, you still require Resources. Also, don't forget that your Dragon's level can't be higher than the level of your Character.