I have this file for example I want the dimension to be smaller by

I have this file for example
I want the dimension to be smaller by 30MM, but I do not want to resize as everything will be smaller, I want just the X axis to be 105 MM instead of 135MM without changing anything else
Is there a simple way instead of splitting and re-merging?

I think I shared the edited file via dropbox with you. I opened it in Cura and changed the X parameter to 105mm but left the others at 135. Saved as STL and done.

No no, if you change the X parameter as is to 105 then everything alongside this axis will be changed, I want only the length and not to alter the corners https://www.dropbox.com/s/kj8ziouv8hz32f7/Screenshot%202018-09-09%2023.33.11.png?dl=0

@Roni_Segoly I Just checked it - X (excluding the corners) is 105 mm.
If you do not have the original file - splitting and merging is quite a quick way to achieve your desired outcome.

in inkscape click until you can edit the nodes. Then find a section of length you want to reduce and shift all the nodes over by highlighting and using i believe shift(if not try ctrl or alt) so they only move horizontally. used this to shrink an item without changing all the detail size of the tabs on the part.

One of the free STL editors should work for this.

The process will be,

Model a new part in whatever CAD package you are familiar with. This part is going to overlap with your original STL so that you can subtract it’s shape from the original shape.

Open your existing part and the new subtraction part in your STL editor.

Then you will do what is called a Boolean operation where the geometry of the new part is subtracted from the original part.

This will leave the dimensions and shape of the original part as it is, with only the modification that you want.

Actually looking at you part now, subtracting on its own won’t work. Split and re-merge will work better.

However the geometry in that file is not great. If I were you, I would re-model it from scratch.

If you plan on changing the length of the joining bars in the future, model it well so you can adjust the overall geometry with a single dimension.

You can’t do this with an STL because it is just triangulated points. If you want to easily change it, you need a parametrically modelled file.