In order you reboot a teammate you will need to be playing a game mode other than solo. If one of your team gets eliminated you will have the opportunity to pick up their reboot card. Act quickly though as the reboot card will disappear after 90 seconds.

Once you have the reboot card the closet reboot van will be marked on your map and you will be able to use the reboot card to revive your eliminated teammate.

If you are later on in the game with a smaller circle there may not be a reboot van in range so you wont be able to reboot your teammate.