First, find the person whose pass / wristband you wish to change.
Click on the pass / wristband number that they currently have as shown in blue under their details:
(Alternatively you can use the 'scan only' mode and scan or type in the pass serial number)
On the next screen you'll see information about the pass / wristband and the person it's linked to.
You'll also see a green button labelled "Swap for another wristband/pass"
Click this to enter the number of the new pass you wish to issue to this person:
As you can see from this screenshot, by default the old wristband/pass cannot be reused. If you want to reuse it (i.e. you might give it to someone else later on) then untick the box.
Click 'Save' and you have now swapped the pass / wristband.