How do i fix this model's arms?

I recently extracted this model from CS:GO, i turned it into a obj file and i used mixamo to rig it because i wanted to use it on VR Chat, it works, but his arms are messed up, i’ve been searching for a way to solve this problem but i couldn’t find any answers, does anybody know how to fix this issue?

Imgur

Imgur

I’m no expert but it looks to me as if you’ve assigned the wrong boneweight to those vertices of the upper arm/shoulder. If that’s the case, load the model into Blender or 3ds and adjust or re-paint the Bone weight of that area.