I am creating a homebrew weapon that can cast spells.  How do I change the spell casting modifier for the spells? It defaults to Intelligence but I was looking to make it Charisma or even Constitution. 

This is because the homebrew creator enforces the Magic Items rules. For magic items that allow you to cast spells, if have a spellcasting feature or Pact Magic feature then it uses the spellcasting ability associated with that spellcasting feature or pact magic feature. If you have both features due to multiclassing then you get to choose. If you have multiple spellcasting features, due to multiclassing, then you can choose which one to use.

If you lack a spellcasting feature or pact magic feature then you basically have no set spellcasting ability and treat spells from the item as if you had a spellcasting ability score of 10. This means spell attacks would be 1d20 + your proficiency bonus + 0, and the Spell Save DC would be 8 + your proficiency bonus. 

This is clarified in the rules on Magic Items. Chapter 14 > Magic Items > Activating an Item > Spells.

A magic item, such as certain staffs, may require you to use your own spellcasting ability when you cast a spell from the item. If you have more than one spellcasting ability, you choose which one to use with the item. If you don't have a spellcasting ability -- perhaps you're a rogue with the Use Magic Device feature -- your spellcasting ability modifier is +0 for the item, and your proficiency bonus does apply.

As an alternative to relying on the spellcasting ability of the wielder, you may have a spells use a set attack bonus/save DC when adding the spell.