problems installing a new theme, newbie!

this is a repost, no one seems to want to help me in the themes and templates section 🙁
Hi all
i have been thrown in at the deep end, the guy that made our site has left the company and refuses to help me without charging extortionate rates. my knowlage is very basic so a please dont make your resonses to techy, although all help is appreciated.

ok so my problem is; our site http://www.fireguardservices.com was working perferctly, then i updated wordpress to the lastest version. now the theme we have installed doesnt work properly (i have managed to get the home page to display properly but thats it!). there is a new version but i dont know how to install it, i have tried downloading the theme and uploading it using the /theme-install.php?tab=upload but it wont work.

thank you in advance for your help