BACKUP YOUR FILES AND DB FIRST!!
I don't believe there are any db changes, you 'should' just be able to copy all tb1.0.3 files over your existing site. If you using the default theme, and you've modified it, then don't copy over the theme. I would try this first. If that fails then try this...
Step 1) Backup your files and DB before doing this!! ALL of your files, and your datebase via phpMyAdmin, a FULL backup!!
2) Delete all folder except:
/config
/img
/modules (except the modules you have installed. In other words delete all core modules)
/overrides (which is probably your problem so i dunno)
/theme/.
3) Unzip the tb1.0.3 file to your computer, and delete /install and /img folders. Then rename admin to your old admin name.
4) Upload the rest to your site, coping over anything that may exist, which shouldnt be much
5) Edit your /config/settings.inc.php file, change the "define('TBVERSION', '1.0.1');" to "define('TBVERSION', '1.0.3');"
6) using phpMyAdmin, modify your tbconfiguration table changing PSVERSIONDB to 1.0.3 and PSINSTALL_VERSION to 1.0.3 (they both should say 1.0.1)
7) goto you admin url, login, and check your settings just for fun, they should all be the same.
8) enjoy 1.0.3
I have to say though, that doing a clean install of 1.0.3 is the better way to go, then you start nice and fresh. All you do is copy over your theme, and your img folder, and install any of your modules you need again. You can 'try' the first way, but this way is better in the long run. Of course you would need to import your customer info, all your product info, ect, but it can be done (and is out of the scope of this writing)