Assuming this happens no matter if you turn it on or off and do other things, which would mean it's a hardware issue, usually the only way to fix it would be to take it apart.
There's some things you could try though, like a compressed air can directed towards the little edges of the button, or very little acetone drips through it while the system is turned off. Basically the idea is it's probably dirty.
They would most likely sell them, but I can't guarantee it would work. If you fiddle with the button does it stop being held or not? By held you mean physically and it's not popping off?
So it's at the same height and it clicks well similarly? If so what do you mean by held, it's always in the clicked state physically or it's normal and it's only in the games that it's held?
The two possibilities are either it's dirt depending on your answer, or the button is broken and needs replacing.
Mb. Not it's nor physically held down, it's just while I'm in games and such. Due to it though, luma3ds won't stop opening the config menu unless I boot into safe mode lmao
If not and it also clicks normally as well, I guess you could try all these things, though your chances may vary. A compressed air can, acetone, or WD-40 all could be purchased from most places like Walmart or hardware stores. The two latter ones are just liquids that help clean things so it's simple enough.
You could first try blowing short bursts into the crevices of the button though while in a game, if you see changes then you can be sure such a fix would work.
I was about to say you could use nail polish as well but it also uses acetone, which I just remembered would not be a great idea through the plastic of the 3DS.
So go with either Isopropyl Alcohol (similar to acetone but good for the plastic) which should be available in the same places but also most pharmacies, or WD-40, if the air can doesn't do the trick.
Good luck anyhow, though I wouldn't be too hopeful.
Well first remove any piece that is blocking you from spraying it, usually it comes packaged like that.
It's as simple as pulling the trigger, just don't shake the bottle nor tilt it downwards. If you tilt it downwards, liquid may come instead of the air. When spraying you need to do it in bursts of a max of 2 seconds. It gets icy and vapour forms on the outside but don't worry about that.
It should've come with a little plastic straw thing, you can attach it to the spray and then use it on the 3DS. Again keep the bottle upright and not tilt it down, hold the 3DS towards it, pull the select button slightly to the side and then spray short bursts through the crevice.