I wanna upgrade front mech on my bike but I have no idea which mech I need to buy? Looking at buying XT. The bike is 3×9 but do I need top swing, conventional? What does this mean? :\

Top swing is where the cage and pivots etc sit above the band.
Conventional swing is where the cage and pivots sit below the band.

Thanks. What about direct mount?

What about direct mount?

Attaches directly to bolts on the frame, or to a plate held on by the bottom bracket.

Is the bike a hardtail or full suss? I learned the hard way that you need to buy front mech for the correct chainstay angle. I think the main options are 63-66 degrees and 66-69 degrees.

Hardtails usually have a smaller chainstay angle (angle between chanstay & seat tube). You can measure using a protractor (I downloaded a tiltmeter app on iphone and it measures the angle perfectly.

If you dont get the correct angle mech, you might not be able to get it to work on all 3 front rings. It affects the height of the cage compared to the chainrings and it can rub.

Unfortunately I bought a 66-69 degree front mech and now I cant use the big chainring. The cage hits the chainring when shifting over.