You can change your class for any character in Fire Emblem Warriors when you have met the following two conditions: that character has reached level 15 and you have at least one Master Seal.
When you’ve met both requirements, go to the Crest Market, select your character, and then select the Boost option. The first crest available will be to change your class:
Master Seals can be found in certain maps after completing specific scenarios. If a map has a Master Seal in it, it’ll be displayed in the bottom-right of the scenario selection screen as shown here: