#1 - Go to Personnel Office
#2 - Scroll all the way down to 'Cabin Crew' (or click 'Cabin Crew' from the overview at the top part)
#3 - Click 'Balance staff between bases'
#4 - Click 'Save'
It will only move the amount of people that are missing. So if base 1 is missing 5 people and base 2 has 50 extra people, it will only move 5 from base 2 to 1. After clicking 'Balance staff between bases' you can see the numbers in the staff amount input fields change (the ones you can change with the arrows) .. you may then adjust those manually too, but you need to remember to click 'save' as the last action.