you want to upgrade to joomla 1.0.15 or migrate to 1.5?
To upgrade to 1.0.15 is very easy. Just go to
http://joomlacode.org/gf/project/joomla/frs/ and download the file named
Joomla_1.0.13_to_1.0.15-Stable-Patch_Package.* and just unzip it over your current site (it will overwrite all the old files) . Unless you have messed manually with the core php scripts of joomla ,you won't have any problem.
If you want to migrate to joomla 1.5.3 things are a bit more difficult/complex.
If you have any modules/components installed you must find their appropriate versions that are for the 1.5. You won't be able to use your 1.0.x templates to joomla 1.5 though.
All of the
categories/sections/articles/menus can be migrated without loosing them. You'll have to re-setup the rest of your site though (modules,template,plugins,components etc).
Two question that will affect your desission are
1.Have you installed many extensions in your 1.0.13 site and
2.Have you spend much time customizing extensions/templates?
I hope I helped a bit
