How do I upgrade from v1.5.1 to latest release?

Hi,

Are there any step-by-step instructions on here for how to upgrade OC from one version to another? I have a client running OC v1.5.1, and I would love to upgrade their install if possible, to make use of new functionality and new extensions, but am worried I will break things if I do.

I don't know if there is an automatic upgrade method, or if I can go direct from 1.5.1 to the latest version.

Any and all help greatly appreciated.

Thanks in advance.

John