You can only start a queued goodybag early when you have less than 100MB or 100 minutes remaining on your current one.
Keep using your goodybag as normal, then once you fulfil either of these criteria, you’ll see a button on your dashboard to “Start my next goodybag now”: http://giffgaff.com/dashboard
Or call 43430, press option 2 and follow the directions to start the queued goodybag early.
You should reboot your phone to force the new goodybag to kick in.
Note that you will lose ALL remaining allowances on your current goodybag if you choose this option.