I am really enjoying printer. As I play around I am noticing I’d like to modify a part to change it interior size to suite my needs. Can someone tell me how I can do this. Is there a free CAD program I can use for this type of work>
You could try the “personal” version of Fusion 360 or FreeCAD or Blender or openSCAD, depending on what you’re comfortable with and what OS you’re using.
How would I know there are errors. These are design modifications to meet my spects.
Because something like OpenSCAD will pop up an error box saying “mesh is not closed! unable to convert!” when you work on somebdy else’s STL file 
OpenSCAD is picky about imported files—that’s one of two reasons I listed it last, along with its peculiar programmer-friendly interface. 30% or so of the time, it’ll throw fits when you hand it a file created in something else, and sometimes you never do find out why.
As for MeshLab, it has an entire submenu dedicated to fixing meshes that aren’t quite up to standard. There are plenty of guides on how to use it scattered across the Web.
Tinkercad can do some stuff. It’s limited but works surprisingly well. Easy to learn.
If it can import your stl…more complex ones won’t import.
-S