Hi,
I can’t find any replacement speed switches online but here’s the ifixit [[Topic:KitchenAid KHM512ER]] guides that may help you to dis-assemble the mixer so that you can see what the problem is.
If it is the selector switch, then perhaps there may be some information on it which may help you to find a replacement part. Use the make and model information (if any) that is on the switch in the search term when searching online.
I suggest that you don’t put “KitchenAid” in the search as this only leads to results for suppliers of complete mixers and not the constituent parts.
Most appliance repair services should be able to fix it but the problem may be a lack of replacement parts and also that the repair labour cost is probably more than a new mixer so reputable repairers would decline to repair it. Given the cost of the appliance it is doubtful that the manufacturer has even provided any, as shown in this [https://www.searspartsdirect.com/model/5dg9pvh84z-000593/kitchenaid-khm512er0-small-appliance-parts|link]. Only the user manual, the beaters and the switch knob are available.