If you are upgrading using the installer file follow the instructions given in Section 3.2, “Installing Under Windows”. There are is no need to remove your current installation.
If you are not using the installer file remove the current MySQL GUI Tools directory, and extract and install the new version as described in Section 3.2, “Installing Under Windows”.
MySQL Workbench is not included in the bundled MySQL GUI Tools download so must be installed separately. To do this, find the link to MySQL Workbench at http://dev.mysql.com/downloads/. After downloading, install this file as described in Section 3.2, “Installing Under Windows”.