upgrade to 2.9.2

but how do I know which I’ve altered I’m using the Atahualpha theme

The WordPress upgrade won’t do anything to your theme unless the theme or the changes you’ve made are not compatible with WordPress 2.9.2. You are probably alright on that score.

If you’ve made changes to the WordPress core files or the default theme, then those would be lost. Determining which files you’ve change might be a huge task if you don’t know how to use diff or other tools. You’d probably have better luck upgrading and rolling back if something bad happens.

Backup your files and database for your current installation before trying to upgrade.

http://codex.wordpress.org/WordPress_Backups
http://codex.wordpress.org/Backing_Up_Your_Database
http://codex.wordpress.org/Restoring_Your_Database_From_Backup

Then give the extended upgrade a try.

http://codex.wordpress.org/Upgrading_WordPress_Extended

Don’t forget to disable your plugins and switch to the default theme before the upgrade. This is a just-in-case and you can re-enable them after the upgrade.

If something goes wrong, restore your file and database backup to get back to where you were.