All you need to do is select the nodes of the path and move them accordingly.
The Editing Paths section of this Inkscape Tutorial details how to edit paths and the keys needed to select multiple nodes, constrain the angle of movement etc. in Inkscape.
Since this is intended to be laser cut for a specific material you probably need the notches to be a specific width. It may be easier to remove the current ones and create your own at the intended width. You can do this in Inkscape using Boolean operations which are described in the same Inskcape tutorial. Just delete the existing areas and create a rectangle at the size you need and use the boolean operations to 'cut' them out of the main shape.
The quick examples I've shown above are from Adobe Illustrator since that is what I use, but the concept is the same for any vector editing software. This is basic stuff so If you have any trouble just search for tutorials relating to the specific software you are using.