Upgrading From Vagrant 1.0.x
The upgrade process from 1.0.x to 1.x is straightforward. Vagrant is quite backwards compatible with Vagrant 1.0.x, so you can simply reinstall Vagrant over your previous installation by downloading the latest package and installing it using standard procedures for your operating system.
As the backwards compatibility page says, Vagrant 1.0.x plugins will not work with Vagrant 1.1+. Many of these plugins have been updated to work with newer versions of Vagrant, so you can look to see if they've been updated. If not however, you will have to remove them before upgrading.
It is recommended you remove all plugins before upgrading, and then slowly add back the plugins. This usually makes for a smoother upgrade process.
If your version of Vagrant was installed via Rubygems, you must uninstall the old version prior to installing the package for the new version of Vagrant. The Rubygems installation is no longer supported.