As above stated maybe order a new sim card from giffgaff but if you can't wait and ur current sim card cannot be converted into a nano sim by pushing it out then take it to a mobile phone kiosk /shop and hopefully they can cut it down to nano size for you.
If you take out your sim please observe whether the sim is 3 in 1 or not.
Most of the sims no a days are 3 in 1.
See the attached photo.
If you didn't have a 3 in 1 sim card,
You can do 2 things to get it done...
1) Sim Cutting
You need to try Ebay for sim cutters. Just under 2£ you will get one. Or try any corner repairing shops will certainly do help for free or may take little money!
2) Sim Swap
You can now activate a new sim if you loose one or it is damaged, or you need a micro or nano sim, by doing a sim swap.
Log out of gg, then click activate a sim, enter the 6 digit code,
then select i have a gg account, then click sim swap. All your account details, credit and goodybags and number will be swapped to the new sim.
Please note, that sim swaps stop between 10 pm and around 6am.
Note, there is no need to top up a sim swapped sim.
You can also see a youtube video here. http://community.giffgaff.com/t5/Blog/How-to-use-SIM-swap-to-transfer-your-number-from-one-card-to/b...
Your best bet is to take your sim to your local phone shop and they will cut to to size for you for a small fee (or if your lucky they will do it fo free)
Order Your Free New 3 in 1 Sim Here
When your new sim arrives do a sim swap via this link
You will not lose any credit or active goody bag during this process.