Sharpen the inside edge using an angle grinder with a 120 grit flap disc. Sharpening this side allows for the tools use whilst standing tall and not having to bend too far at the waist.
A metal grinding wheel may be needed for shaping the edge if it is in bad shape.
|35 - 40º
If the original angle is different, it is best to match that, but the angle should be in the range of 30 - 45º.
I recommend sharpening the edge closer to the user as it makes the angle for using it easier, i.e., the user can stand up straight. If the other edge is sharpened, the user will have to bend down to use it.