How to erase materials that connect to other materials (Picutures)

I am trying to erase the top portion that is sticking out from the top thing. Whenever I delete it it also deletes the base. Can you help me with step by stem procedures?What Fabio Gave me CAN OPEN IN 14.skp (135.9 KB)
If you want to know exactly what is it that I mean, please open the file below, this one can open in skectchup 13, not just 14 actually

Open the group with the top portion, triple click to select everything in that group, rightclick and select intersect with model, delete the unwanted part of that group…

If you mean that you’d like to end up it something like this, then edit down to the context of the part’s faces - set View > Component Edit to NOT show other components, so you can see want you are doing…
Now select all, context-menu > intersect with model.
Erase the upper [unwanted] geometry.

Couple of points…

You are modeling at a very small size - SketchUp can’t cope with geometry <1/1000" and so followme, intersect etc can fail to make all expected edges/facets…
Scale up x100, do the changes and then scale down…

Also the part looks like it could be the exact match to its neighbor - using component-instances would mean that editing just one edits both [or more]…

The edited part is no longer solid with it’s top cut off…
If you need a solid [e.g. for 3d-printing] then you need to reverse engineer the surface needed to close it by intersecting the horizontal [green] part to get the cutout needed - remember the re-orient the top surfae so all faces are ‘outwards’…

And will in this case…

Thank you this helped out a lot, unfortunatly my partner messed up on the dimensions of this so I now have a new problem, can you help me, Is there an way I can properly fill in this gap and still connect the two materials together so it will print as one piece?

What Fabio Gave me CAN OPEN IN 14.skp (120.8 KB)

Here’s one solid version…

What Fabio Gave me CAN OPEN IN 14_b.zip (48.8 KB)