How do I bring back old pc characters in a new campain

I'm playing with my family some homebrew and started a new campain in the same world as the last campain.

Plater 1 player a fairy mage with a lil druig powers

Player 2 player a dwarf with a love for gold

Player 3 player Robin Hood

Also the new campain takes place 500 years in the future where after a war the whole world is changed

Just asking for advise from some Dm's with more experience