To equip or change a Spell, navigate to the Hero screen and select the Spells. Select the Spell slot you want to change on the left side of the Spell screen.
A full list of all the Spells you've collected will be displayed in your Spell Vault, including Spells you've collected using your other Heroes, even if they are from a different Hero Class.
To change your Spell, select a Spell slot on your main Spell screen. This will then display all your Spells from your Vault.
To quickly equip a spell, you can tap the Up arrow button on the right side of the Spell. You can also unequip a spell by selecting the unequip button on the bottom right-hand corner of your currently equipped spell.
To compare a new Spell to your currently equipped Spell, select a Spell from the list on the right. Then press the Spell slot number next to your currently equipped Spell (in the example above ). This will then will show you the details of both Spells side by side.
Note: If you equip a Spell with a higher level than your Hero's level, the Spell will only have the effects as if it were the same level as your Hero.
While this Poison Skulls spell is Level 20, if I was to equip it to my Level 19 Hero, then a yellow down arrow will appear next to the Spell's Element icon and it will display as a level 19 Spell when you view it on your level 19 Hero.