Have you read these instructions: http://learn.elgg.org/en/2.0/admin/upgrading.html ?

I'll take a look thanks, does anyone know of a plugin that can move all the data from an old Elgg to a new one?

There's a plugin that seems to be able to clone an Elgg installation: https://elgg.org/plugins/2402241. But I've not tried that myself and I don't know what exactly it does. There are some other plugins but partly they only backup the database and partly they are already quite old (i.e. possibly not working anymore on recent versions of Elgg).

But it shouldn't be necessary to use such a plugin in your case. If you only want to update your site (but not move the site to another server or change the location on the server), you only need to follow the instructions given in the link Juho posted. You would update your existing installation and not making a new installation.

If you want to move your site to another location, you can follow the instructions given at http://learn.elgg.org/en/2.0/admin/duplicate-installation.html. But this will only move the site and not change the version of Elgg. It would be a separate task to update to a newer Elgg version (as described in the other link). You could either update your site before or after the move. Regardless if moving your site or only updating it, you should make a full backup (install directory with all subdirectories, database and the whole data directory) to be able to restore the site if the update goes wrong or if something fails to work out right on the move. Then you can just try it again.

Any plugin (like the one I linked above) is quite likely requiring the same version of Elgg to be used on the old and new location. Therefore, you would have to update Elgg to a newer release in any way anyway. So, I would suggest to just try it out. You could make a "test run" for example by setting up a Elgg test installation on your own computer and just try out how the update progress works out. This way you can't damage anything.

When upgrading from Elgg 1.8 to 1.12 you need to do it in several steps: from 1.8.10 to the latest 1.9.X, then to the latest 1.10.X, latest 1.11.X and only then to the latest 1.12.X version of Elgg. You should also make sure that for any 3rd party plugin you use compatible releases for the new Elgg versions are available (plugins for Elgg 1.9 should almost all work also on Elgg 1.10 - 1.12 and plugins for Elgg 1.10/1.11 will surely work on Elgg 1.12). For testing plugin compatibility it's also useful to try it first on a test installation.