Basically; I am trying to import a plate mesh I made in Blender to Roblox, but it has an exaggerated amount of triangles (Literally 1,507,328 triangles), with the maximum triangle limit being 10k. I want to make the triangle count lower, but I have no idea how. Here is the .obj file: plateTest.obj (272.7 KB)
And here are two photos of it: Imgur: The magic of the Internet
Keep in mind I have no idea how to use Blender, I watched a tutorial to make the plate; help would be appreciated.
Sadly you can’t decrease the amount of triangles unless you create the blender model again 
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Creating it again would make the amount of triangles the same, though. I recently made a cup and it was fine. I will try lowering the smoothness.
I lowered the smoothness quality to 3 and it’s fine, now it has 23K triangles. I just have to separate it.
Oh I had no idea that you increased the smoothness
Use the Decimate modifier set to Planar to remove a lot of that redundant geometry.
I’ll also mention, because you said you’re new, that there’s a huge difference between modelling for renders and modelling for games. Rendering tends to have very high fidelity, big textures, finer details, etc. Whereas games are made to look high fidelity (or a certain way) but actually are not at all (in most cases). What you have there is a beautiful asset for rendering, but it’s not at all meant for video games 
Basically a simple flattened 12-sided cylinder mesh with a bit of an indent is more than enough for most cases.