I currently work as a courier driver and sometimes when I'm delivering parcels to a customer's house their dog comes out and touches my leg. Do I need to do gusl or have a shower when I get home to pray namaaz or can I just change my clothes?

If the dog licks you or is wet and touches you, then you must wash that area the dog licked since it is najes. Then you can pray. But no ghusl is required.