How do i rotate a players arm without it breaking off?

Each limb has joints that connect to it from the Torso. The Torso contains all these joints, and they are called Left Shoulder, Right Shoulder, Left Hip, Right Hip, and Neck. If you want to rotate a limb, you have to edit its C0 and/or C1 properties. You can do that like this:

local Left_Shoulder = Torso:WaitForChild("Left Shoulder")
Left_Shoulder.C0 = CFrame.new(-1.5,1,0)
Left_Shoulder.C1 = CFrame.Angles(math.rad(45),0,0)

Note: Those CFrame values don't always have to be like that, you can change them however you like

Hope this helped!