changing spells

Perhaps this is an obvious question, but I can't find an answer anywhere.   Can you change the spells of an Archmage or Lich from the ones listed in the Monster Manual?   I'm looking to 'beef up' an archmage.  

You can make your own monster using the homebrew monster tools. You can start by copying an existing monster and then modifying the spells.

The DMG has a section for this: https://www.dndbeyond.com/compendium/rules/dmg/dungeon-masters-workshop#CreatingaMonster

I was pretty sure it mentioned changing spells, but not sure where.

Thanks everyone!  That really helps!