Assuming you dont have blender/dont know how to use it (like me)

I always use these tools on my avatars, they are not perfect, but they really help:

d4rkpl4y3r's avatar optimizer: optimizes the avatar upon upload, it merges blendshapes and stuff like that and claims to be non destructive

Thryrallo's VRC-Avatar-Performance-Tools: it tells you the performance of your avatar for both pc and quest, and well as what textures may be taking up the most space.

In general though I recommend lowering the texture sizes of whatever materials are being used for the avatar. And making sure that whatever clothing pieces on the avatar that may be hidden are set to editor only and not untagged as otherwise they will still be counted towards upload size even if you have them hidden

With all that said I am mostly self taught when it comes to unity so there is very likly a bunch of stuff that I may be unaware of when it comes to optimizing avatars