Change definition query

I have three feature classes that have multiple features in each layer. Each feature class has a field called parcel_no. I turn different parcels on by a definition query on each layer such as parcel_no = 1. This will show all three layer for parcel 1. I would like to run a script that will change the definition query for all three layers, say from parcel_no = 1 to parcel_no = 5. New to python and need to be pointed in the right direction.