Daniel, You can find out about upgrading Elgg in the documentation here. http://learn.elgg.org/en/1.10/admin/upgrading.html

First and most important; back up everything!

I did upgrade from 1.9.8 to 1.10 and it went fairly smoothly. A couple of tips I can give you is:

If you are using any plugins in addition to the core ones, download the latest plugins for the version you are upgrading to. A lot of the plugins that work with 1.9 also work with 1.10.

Deactivate any existing plugins. Old plugins you are replacing, delete the current folders and upload the updated plugins for the current version. I also always flush the cache.

Activate the new plugins one at a time and deal with any issues if there are any.

Dan

Do not skip Elgg 1.9. Upgrade first to the latest 1.9.x version and after that proceed to latest 1.10.x.