Remove tv plaform on metal guides from old tv armoire
If there are no release tabs on the slides edges(that are usually pressed to separate the two parts than you may have to remove the wood shelf that is attached to the slides.
Extend the shelf out fully and look underneath where it is attached to the slides. Look in the center of the slide track for Philip head screws going into the wood.
Also while the shelf is extended, look at the lower slide that attaches to the armoire shelf. Look for screws along the center of the track.
Sometimes the release tabs are not obvious and don't seem like tabs at all. They are usually located at the back of the top (extended/shelf) slide or the front of the lower slide. Usually along the edge on both.
Also look below the compartment where the TV is located. The lower slide may have been bolted through the Armoires' shelf (not likely, but possible).