Have a look here "My giffgaff">Spread giffgaff to order a new sim
The micro sim will probably be encased inside a normal sized sim and you'll have to pop it out. When it arrives perform a sim swap and your account details will all be transferred seamlessly over to the new sim.
For guidance on how to do the swap see here What is SIM Swap ? - The giffgaff community
hi there should be a pop out in the one you have but if not order your sim here:https://giffgaff.com/orders/mgm
sim swap, please see here:http://community.giffgaff.com/t5/Using-giffgaff/What-is-SIM-Swap/ta-p/7320084
this enables you to keep your number (and credit)