I think that innerdream is talking about deleting an individual instrument preset and Peter is talking about deleting a 4-layer Multi.
There is an
invisible Delete (X) button to the left of the preset name. Hover over it to see it. Then you can delete the preset. It looks like you can delete the factory instruments too, so be careful.
>>>
https://i.imgur.com/oEPCNWU.png--------------------------------
The Developers really need to fix that invisibility and document it in the User Manual. Oddly, the screenshot in 3.1 does show the X but it is not described.)