Desolder the larger gray motor wire running to the arm to be replaced

Desolder the larger white motor wire

Desolder the larger black motor wire

Desolder the smaller black LED wire

Desolder the smaller white LED wire

The marked wires are for the right arm. The wires to desolder when replacing the left arm are the mirror image of the ones shown here.

Don't forget that plastic has a tendency to melt when exposed to heat! I may or may not have disregarded this fact and put a mark on my drone...