Updating a modded Xbox360?

I bought a modded xbox 360 from craigslist about a year ago along with a bunch of older burned games.. A few months after that around the time ins creed 3 game out.,i bought from a differant guy several current title games. They wouldnt play. I contacted him and he told me he could up "flash" my system for 20 bucks..... long story short,i dropped off my machine,ten minutes later i pick it up,and everything plays perfectly..... I have since found out how to download torrents with ISO game files. I burn them to the dual layer disks,and they wont play (disk tray is open msg)... I have searched message boards and ect.. and i believe i need to 'flash' my machine again to play the newer title games.... the guy I went to last year is unreachable,and i cannot find any other place close by. I am pretty smart,and was wondering if someone could give me some direction or a link to some directions. I dont ever play on xbox live,and i have never connected my unit to the internet. Please help.