How can I decrease the thickness of a wall in an STL file?
The process is rather simple:


import your model into either a modeling software (e.g. Blender et al.) or a CAD-program (e.g. fusion360, Design Spark Mechanical et al.) that can import and export STL.


if needed, convert the STL into a useable model with your chosen program's functions or switch to edit mode.

select inner walls and extrude them the desired amount.
export as STL again.