Editing imported STL files. (Geometry mayhem?)

I’ve imported a few files, and some I have had joy with adjusting, however I’m now having trouble with this model.
Once imported, it is an utter mess of lines, which makes it near impossible to edit.
Here is a picture once imported.

I’ve added an extension called cleanup, which then leaves me with this

However, I still can’t edit the file due to the mess around the sides, or edges (like removing some parts). Is there any way to clean these as well?
I am a newbie to Sketchup, so I might be missing something obvious, but for the life of me, I’m stumped.

Many thanks

Make the upper face a group then delete the rest of the geometry then get inside the group and push pull it.

Took a little while to get there, but I found it! Massive help and definitely makes it easier to change.
Thanks bud. :smile: