I would suggest making sure that everything works okay before introducing a significant change such as a FW update. Also, make sure that you know which FW versions you currently have on the AC and the RC before you do anything so we can better help you with issues.
If however you are on the brave side and still want to make a change, DJI has historically maintained an archive of FW versions on their site even though they don't always show links to them. For archived FW versions directly from DJI, follow the links in this post.
You can also always go to DJI's product page if you only want the latest. [P3A Downloads Page].
Note: If you see the "For Your Attention" pop up from DJI telling you that the P3A is no longer in production, just close the pop up.