Which one is the latest version of Asterisk?
Asterisk 1.8 or 11?
I have asterisk 1.8.
If 11 is the latest version how can I upgrade it?
Do I have to re install asterisk again?
Asterisk 11 is the newest Long Term Support Release.
Asterisk 1.8 was a Long Term Support release as well (and still has support
Asterisk 10 is a standard release.
More info about the version on https://wiki.asterisk.org/wiki/display/AST/Asterisk+Versions
Like you say: you have to reinstall. Although your configuration files will not be overwritten (as you don’t do a ‘make samples’, that will overwrite them.)
You have to download the source and compile Asterisk again.
You also need to read UPGRADE*.txt, and make appropriate adjustments to the configuration.