There are occasionally times where a GM will change the BP/Karma or maximum Availability you're allowed to use while you're still creating your character. Rather than having to recreate everything, Chummer lets you change these limits while still in Create Mode.
Open the Special menu and select Change BP/Avail Limit.
The Choose BP window opens again, letting you select the new amount of BP or Karma you're allowed to use, as well as the maximum Availability for items. When the values have been corrected, click OK.