It is good practice to create an archive copy of main Revit project files at regular intervals, or before doing a major change to the model.
WARNING:
DO NOT COPY and PASTE, or MOVE A REVIT CENTRAL PROJECT FILE
Revit maintains the path within the file, if you copy or move the file Revit will get confused. You must do a Save As from inside the project file.
There are 2 methods to Save As, one if you don't have the file open, the other if you are in the open file.
SAVE AS BEFORE OPENING FILE
This method is described in:
PROTOCOLS > MODEL MANAGEMENT > MAIN CENTRAL FILE MANAGEMENT
SAVE AS AFTER OPENING FILE
This assumes you are in a local copy of the central file (your name will be appended to the central filename as shown below).
Sync to Central so your local file is updated.
Go to the start up sheet view. Edit (or add) text to notes section describing file as an archive.
From 'R' top left corner, pick Save As, then Project.
In the Save As dialog box hit the Options... button. Tick Make this a Central Model after save.
(this step is critical - if not done you are merely creating a new local file)
Browse to the Archive folder. Create a new folder, use iso date as the name, add a description if appropriate.
Change the name of the file, remove your name and replace it with an iso date.
Hit Save button.
Close the file.