Note: Always make a backup of modified files before changing
Go into the directory where you have installed the game. Find the file called "banditsdataprofilesGARAGE.XML" and open it with notepad. Edit the lines as you want. For example, if you want all weapons available from the first mission, find "Mission 01" in the file and change all "no" values to "yes". Start the game and modify your weaponry.