This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for The Good Ol' Days side quest.
Time frame: From Majin Buu Reborn - Episode 2: The World Tournament Begins! to Episode 2: The World Tournament Begins!
Reward: 216,397 EXP, 3x D Medal, Celebrity Fan Photo
Where to start: Far Southern Islands Area, as Gohan.
To start this mission, you need to go to the World Tournament Arena. There you can find a man who will order you an additional story.
This mission is very simple. You only need to kill the robot and return to the quest giver. It won't be a problem for you.