As version 1.6 upgrade to version 2.0?

Not yet. You are supposed to install 2.0 from zero. However there are some discussions right now on the developer list to propose a little bit later an upgrade path that may work but may still need some manual work. But it seems that it is not as easy as it seems as there are some major differences between 1.6 and 2.0 that may not have an upgrade path (going from asterisk 1.4 to 1.6 for instance is not supposed to be done by upgrade).