Newbie - Crop Existing Design

I am completely new to 3D Printing and CAD. I’m printing tool grid plates for my toolbox which are 6x6. I’m at the point where I need to print filler plates for around the edges. I have the STL files but for the life of me cannot figure out who to crop the design to specific sizes. It all confuses the hell out of me.

Is there a guide somewhere on how to do this? This is the file I’m working with:

https://makerworld.com/en/models/33118-toolgrid-compatible-6x6-peg-board#profileId-356432