Spell Mastery
18th-Level Wizard Feature
At 18th level, you have achieved such mastery over certain Spells that you can cast them at will. Choose a 1st-level Wizard spell and a 2nd-level Wizard spell that are in your Spellbook. You can cast those spells at their lowest level without expending a spell slot when you have them prepared. If you want to cast either spell at a higher level, you must expend a spell slot as normal.
By spending 8 hours in study, you can exchange one or both of the Spells you chose for different spells of the same levels.