there are 3 ways to do this
Step 1: First way
all you do is put your mouth round the centre and suck in as hard as you can (this may take a few attemps)
Step 2: Second method
first get your sellotape and cut about a inch and a half off
then stick it to the centre not leaving and part uncovered (leave a bit of sellotape for your fingers) and yank it as hard as you can this also may take a few attemps
Step 3: Method 3
anyways what you will need for this step is a vaccum cleaner so this is reerly a last resort because you should have had it unpopped by the first step
what you do is(this one's obvious) find the right attachment to fit the bubble in the centre and set your vaccum to the highest suck it has, then turn it on and just place it over the bubble and just leave it until it comes out!
(this can also be done with a hairdryer)
Step 4: Finished!
sorry about the images my camera wasnt useable or else i would have shown more in them