You should not need to change the oil in the pump. It uses 4 oz. of 15W30 synthetic oil. This oil will not break down and need to be changed as a regular maintenance task. If the pump is working properly and producing proper water pressure, I do recommend that you drain and replace the oil.
If you do need to replace the oil for some reason, there is an oil drain plug on the bottom of the pump. The oil can be filled through a plug on the top. You can view the parts list diagram for your pump in the owner's manual. Here is a link for the owner's manual on this pressure washer in case you need it:
If you do not feel confident in changing the oil in this pressure washer yourself, then you can have it changed by a Sears technician. Here is a Sears Service link that will provide the nearest drop-off locations:
Sears Home Services