If your old SIM fits your new device, you can carry on using it. In this case, you’re free to discard the new blank SIM we sent you, or you can put it safely away somewhere.
If it doesn’t fit, you may be able to snap out the size you require from your old SIM, even if it looks like it won’t fit at first. The following video on multi-SIMs explains how you should do it.
If your old SIM still doesn’t fit, then you’ll need to use the new blank sim we sent you - you'll need to activate this first before you can use it.