How to replace garbage disposal splash guard?

Factual Questions
1

And upside down knife has shredded the rubbor or plastic thingy that attaches to the hole in the sink to stop water from splashing up when the garbage disposal is on. Do I have to replace this from underneath, or is it held in place the the drain opening ring, and therefore can it be replaced from above?

2

I was about to say it depends on the model of disposer you have, but on second thought, I can’t recall one single brand that allowed you to replace the splash guard from above. (Except MAYBE a Tappan/Whirlaway, and I’m not even sure they are still being manufactured.)
On the most common disposer brand, the In-Sink-Erator Badger line, the splash guard also acts as a seal between the sink and disposer. You will have to remove the disposer to replace it. Replacements are generally available in hardware stores where the disposers are sold.
Removing the disposer is fairly easy, the second time :wink: , it just takes a few simple tools, including a sturdy flat-blade screwdriver and a pair of channel-lock pliers or similar device.
You will want to unplug the disposer first and keep the plug where it won’t get wet when you pull the drains loose. Disconnect the dishwasher drain hose, if applicable, using the screwdriver to loosen the clamp. Next, disconnect the main drain either by unscrewing the drain bend from the body of the disposer or by loosening the p-trap assembly. Then use the screwdriver to remove the disposer from the sink. This is accomplished by sliding the screwdriver blade into one of the three metal loops at the top of the disposer and using it as a lever to unscrew the retaining ring (righty-tighty, lefty-loosey). At this point, you should be holding the disposer in your hands. After that, replacing the splash guard is a self-evident exercise. Re-assemble the works in the reverse order and check for leaks before plugging it back in.

Nothing to it. Don’t forget to have a shallow pan handy to catch the miscellaneous drain water.

Also, a new disposer only costs about $50-$75, so if yours is getting old, this is a pretty good time to replace it.