Most of this is fairly easy, the bulk of the game's files are in the open except for the DLC files. A lot of the data is stored in a file format called JSON that are like fancy text files ending in .json extensions. You can use a free editor like "Notepad++" to open and edit these files.
If you open the stock Medium Laser file and change the damage property to 15, you now have a ML that does 15 dmg instead of 25. It's that easy, but I recommend one more step.
Google "ModTek Battletech" and read the guides on how to make your own ModTek based mod. This lets you not edit the stock files and keep them safe by overwriting and merging them with your edits before being loaded in to the running instance of the game.
This is essential for keeping your custom edits between game updates. If you mess with the stock files, and an update changes them all you are out of luck. ModTek's github page has some extenisve tutorials on how to write a .json for ModTek and the mod.json needed for ModTek to load it in.
This is where to start. Try something simple like making a ModTek mod that just alters the damage on a ML, play a skirmish and see if it worked. Once you know you can do that, poke around in the game's folders looking for what you can change or ask anyone here or on the various discord servers.