I had a rear projection big screen for several years. I used to clean the inside from dust and webs every year or so. You may also notice that after you clean the lens and mirror that your picture is brighter. The tv’s are not sealed, because then need a means for heat to escape from the projection unit.
In my TV (it was a magnavox), there was just a large panel that would come of and leave the whole backside open. There were probably a dozen screws to remove, then it just came off and I could just the lenses, the mirror, and the inside of the screen. I used a vacuum to collect the dust to remove as much as possible. Use something soft that won’t scratch the lenses/mirror/screen to dust with. Unplug your TV first, obviously, and let it cool down before you do this.
Not much to it really. If your TV is like my TV, anyone with any mechanical skill could do it. You will be surprised at how much dust and stuff gets in there. When you are done, just put the back panel on and you are good to go. This is a pretty typical procedure, and I would do it every so often.