How to change oil on lawn mower and replace missing spool on gas weed whacker?

If it's a regular rotary mower, vertical shaft engine, then pull the wire off the spark plug (for safety), turn the mower on it's side and remove the square head drain plug and drain the old oil into a proper container. Replace the plug and refill with oil, replace the spark plug wire and that's about it. As far as the missing spool for your string trimmer, HD, Lowes, local hardware, or a power equipment shop sells replacement spools, some prewound with string. Just need to give them the make and model of your machine.