Using Old files with newer version of Joomla

I have had an older website (at least 5 years old) on another account... I have since started a new account, and am building other websites. I wanted to combine all my accounts into one, so from an old account I downloaded the files from an older jumla website. We created a new subdomain of one of my domains in the newer account. That new subdomain is beta.**********.org. If you go to look at it, you see many errors. I am assuming this is because the files are old, and the current load/version of joomla is newer. I am wondering how to go about getting those errors removed... one way is to hire someone to fix them (I do not know how). But I am wondering if there is another option. Since I have not yet cancelled my old account, can I copy from it an older version of jumla and move that over to the beta.******.org subdomain? (Recall, the old website works just fine as it is on the old account.... ).