Go into the Project you are working on, then in Project Manager, right click on the Project, and click Properties.
In the dialog box, on the left hand side, open up P&ID DWG Settings, then click on Paths. This will tell you the Drawing Template file that P&ID uses.
From here, you can use AutoCAD to edit that DWT file, or replace the file with the one you want to use.
When you create a new project, at the first page is a tick box that says "Copy settings from a template project"
No problem, please mark this thread as resolved, unless you have any other questions
Right click on the Project in Project Manager, go to Properties, then expand the section marked "P&ID DWG Settings"
Under that is a section called "PID Class Settings (or to that effect, I'm not on a PC at the moment) and under that is all the settings for each of the elements in a P&ID. When you find the one you want, clicking on "Edit Symbol" does the layer, colors, linetype etc, and then clicking on "Edit Block" edits the actual block.