Yes...it's not a solid piece of wood. The whole thing is made up of layers (probably plywood) with trim nailed on the three visible sides of each layer. The main issue is whether you are confident you have the skills to take apart the trim around the top layer and then remove what is probably a 3/4" layer of plywood without damaging the parts below that you want to leave and not dislodge, as well as not damaging the wall above when you pry the top piece off. This could be simple, or it could be hard (like if the top piece is glued on rather than just nailed on). You simply won't know until you start taking it apart. Since you're asking this question, I'm guessing that you don't have a lot of trim carpentry experience, so it might be good if you have a more experienced friend to help you. But what you are asking is definitely doable. Good luck.

Yes, it can be done. Sometimes there are screws hidden under glued in plugs holding the top piece in place. Disassembly can be "interesting", but it went together in some logical order and can come apart the same way. If the carpenter who built it was a belt and suspenders "screw and glue" kind of guy the disassembly can be even more interesting. You won't know until you try to get it apart.

Pulling it off the wall to see what the construction looks like from the back side can help, as can stripping the paint off to help expose fasteners. (chances are good that it's only held to the wall with a few nails, usually removing the entire mantle in modern construction isn't a big deal.

Thank you for your responses. I think this will be next project attempt. I'll post an update once it's eventually done.

Why do you want to remove it? Whats your end goal?

If you're trying to replace the shelf with a thicker one you could just mount it over it ...basically a overhang shelf instead of removing

Yes, I wanted you place another shelf. That would be great if I could get an overhang in the style I want. I'm going to look. Thanks for the idea!

Its a nice looking mantle, I wouldnt want to do anything that may damage it. Yes, you can put another shelf on top of the one you have, but, what you have already looks very nice and appropriate for your fireplace. Personally, I would leave it alone. Not much gain, from a thicker mantle.

The question i think is “why update the mantel on a non-working fireplace?”

Even thought we don't use the fireplace, I'd still like it to look like I want it to look. 🤷‍♀️

Maybe post an inspo pic of what you want it to look like so people can advise better

Or what bothers you about rhe existing top piece or is it the whole design you dislike?

I agree with the person who said you might cause more damage to the whole thing and honestly even the wall if you try to demo it so adding an overhanging shelf or just leaving it is best

I installed many mantels over the years. If it is well built, it will be glued and nailed together, which means it will likely not come apart easily. I would be prepared to use a Sawzall and then deal with the rough edges when you replace the top.

Yes, it would be best to post an inspiration photo so we can offer the best advice.

Consider having the mantle you want made to go over what is there as a cap rather than a replacement.