Just upgraded to a 65" Samsung TV, Dowloaded the Plex app, but it has the new inetrface I don’t care for.
How can I switch back to the desktop mode to search libraries and navigate through Plex?
You can’t - at least I don’t think you can.
Plex Media Player has the ‘Dueling Banjo Views Thing’ - that you bounce back and forth with using the ‘Full Screen’ arrows. Regular apps - I’m going to assume the one installed on your TV is one of those - don’t have this ‘feature/curse’.
If you don’t maintain the old HDMI cable from the Server to the TV (like I do) you can’t really get PMP on that TV, so you’re stuck with what you have - whatever that is.
If you do maintain the old HDMI cable from the server to the TV - ‘top of the forum/downloads/get an app’ will get you to Plex Media Player - and then you can pick one of the two views - as you figure out where to install the 20 foot mouse extension cable and on which end table you want the mouse on.

My server is ok, it’s attached to a computer in a different room. I kept the same HDMI cable from my previous tv in the living room but it doesn’t look like it prevented the Plex app on the TV from defaulting to this new interface.
Don’t understand why in the world they would force you to use that interface on TV apps.
If you want PMP on that TV you have to plug in an HDMI cable from the server. The app in your TV is what it is - you can’t change that, but I do feel your pain. You, me and thousands of others detest Plex’s App Skills, but our choices are limited.