If you use akeebabckup create a backup site in the subdomain of your main site. You will have to delete the htaccess file to install the backup site and run the update to Joomla 3

Once you backup site is installed in the subdomain on a new database go to

Joomla Update > Options > Change Long Term Support to Short Term Support

You should see the update to Joomla 3.0

If you can't see the update

Go to extensions > Extensions manager > options > preferences : Update caching in 0 hours

Then purged cache and discovered 

In Components > Joomla Update > Joomla 3.0 should be there