There are two components that make up your shower hardware. There is the valve and the trim.
Valve: The valve is the piece in the wall that your water lines feed to. This component is what actually mixes the hot and cold water to control the temperature.
Trim: The trim is the things you see on the tile, this includes the control handle, trim plate and shower head. This component is what allows you to operate the valve and mix the water. Each trim kit is made specifically for the valve.
If you want to change out just the trim you will need to know what valve is in the wall so you can purchase the trim kit made for that specific valve.
What to know when purchasing a new shower valve