Just press \'t\' while in your ship. Then lick the weapons computer text. You should see stats on the right hand side of the menu. There should be a stat for range there. You slide down one stat as many times you like, then increase the range stat. However, the stat you slide down makes it worse, so it is good to pull stats from multiple sources to balance it out. Note that if you have many different sets of cannons, they all have individual stat menus. So each cannon you want to alter must be done one by one.