How to fix "mixing" valve on Temptrol Shower ?

We have a Symmons Temptrol shower control/mixing valve.
It no longer controls the temperature (pressure?) properly and the shower changes temperature significantly when other water is used in the house. Otherwise it still works fine.
I assume that there's a replaceable cartridge or other item in there ? Can someone give me a couple of sentences on how I'd access the unit and what to replace ?
Thanks, Bob

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.