How do i fix this model's arms?

[Senninha]

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?

[url]https://imgur.com/WMwWq9X[/url]

[url]https://imgur.com/AuPXxjD[/url]

[Raban]

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.