If it is currently active then no, you will have to wait until it expires (one calendar month after it started)
If it is a queued goodybag that hasn't started, providing it isn't due to start in next few hours, then you will need to contact an agent and they should cancel it for you.
To contact an Agent go to your My giffgaff page and you'll find links to the Ask an Agent form half way down the left hand side. Here's a link to it in case you're on a mobile - https://giffgaff.com/support/ask"
Once a goodybag is active it can not be changed in any way, neither can it be ended early or run alongside a 2nd bag. It runs the full month and if you use up any of the allowances before the bag's expiry date you will need to use airtime credit at the following rates:
Calls, texts, call forwarding and video calls to giffgaff
Calls to other UK mobiles, landlines and call forwarding (per minute)
10p after inclusive
Texts to other UK mobiles and landlines
6p after inclusive
Voice mail (per call)
8p after the inclusive
2p/MB after inclusive
Free phone numbers (starting 080 & 0500)
However, if you have got a goodybag queued up to start once the current one expires then you can ask for that to be cancelled if you want. To cancel a queued goodybag you will need to contact an agent. It can only be cancelled if there is more than 24 hours left until the activation. Use this link: http://giff.ly/agentscancelqueued
Remember, it can take up to 24 hours to get a reply, so be patient.
Finally, if you mean can you change the goodybag you have got set to recur automatically, then you can do that here: https://giffgaff.com/profile/payment-details
*edit for typos*