Is there a way to take the cantilever top off?

I’m unsure how to cut JUST the hanging part out right now (to make it into a straight block with a slope), every time I try to draw a line on one side of the block it just disappears somewhere and I can never find that line… Please give me some help!

[FInal Model .3dm|attachment]d://vd6odJ1LWBvH5TIyfwN4A6hpgNR.3dm) (14.7 MB)

FInal Model .3dm (14.7 MB)

Sorry, the model is attached here! (In Rhino 7)

Hello - please, in the future, export the parts you are asking about to a new file and avoid posting the entire project if you do not need to.
There are a few ways to split this, the easiest is BooleanSplit (or BooleanDifference) . Select the oject and at the prompt to select the object to split with type in ‘ip’ (for InfinitePlane) and Enter. Select the vertical face under the overhang as the plane to split with, and Enter.