Depending on the type of asset you wish to modify, there are different steps that must be performed and different files to be worked with.
Some stages will apply to all types. This page describes the different things you may want to do with various types.
You have a prop that you want to change the look of, altering either the material its made from, or just the colour of the whole object, or the individual parts it may be made up of.
You have a prop that you want to change it's shape in some way.