This is my first post so bare with me if I word things wrong.
When I first got my panda my sister bumped the back so the right side bumper Is now hanging down about 1cm and the plastic trim has broken,
How do I check If the bumper can just be pushed back into place and where are the best places to replace the plastic trim ?

The plastic trim will be expensive from Fiat but you will (eventually) find parts on eBay.

The trim is retained with push on clips. Look behind and you will see where the captive pegs push through. Item 311491963920 looks correct but you will need to check.
If the trim is broken but otherwise OK, you can stick it back with polyurethane body panel sealant/adhesive e.g. Tiger Seal (other brands are available. )