

For example, to move the arm, you want to select either the upper arm or forearm bone, go to Curve editor, type "rotation" into the curve editor search box and try moving the whole curves up and down. then make sure to turn the IK influence down to 0 for these (otherwise the mixamo animation won't work). If you've rigged the rest of the arms, legs, etc. For example, you can leave the rest of the animation as is, and just control the arm with your IK.

I realize this is an old question, but the best way I found for editing Mixamo's animations are these:
