When you regroup, you shuffle things around and structure or organize them in a new way. After an out-of-control day at camp, the counselors might want to regroup and make a new plan for the next day.
People tend to regroup after some kind of defeat, loss, or difficulty. Armies need to regroup after losing on the battlefield, and football teams will regroup at halftime after a difficult first half. You can also use words like reorganize or restructure, or even mobilize to substitute for the verb regroup.