This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for The Capitan's Comeback side quest.
Time frame: From Majin Buu Reborn - Episode 8: Finale to No time limit
Reward: 238,963 EXP, D Medal, Improved Scouter
Where to start: Southeast Islands Area, as Vegeta.
To start this mission, you must go to Southeast Islands Area. Talk to Bulma - she will give you a new substory.
Your first task is to find seven Dragon Balls. They are scattered on the map.
After collecting the balls, make a wish for the captain to return to the correct body.
When the captain is back in his body, fight him. Defeat Ginyu to complete this substory.