You should be able to easily update your existing version of WEAP to this new version. When WEAP starts, it should find this update and offer to download and install it automatically. If not, go to the menu in WEAP: Help, Check on Internet for New Version. You can also download the full installation from http://www.weap21.org/Download
Be aware of some differences in where WEAP datasets are stored in this new version. The old version of WEAP installed in \Program Files\WEAP21, and kept all areas (datasets) in subdirectories of that program directory. The new version installs into \Program Files\WEAP, and places all areas under My Documents. If you are updating a previous version, it will keep the old directory structure (areas stored underneath program folder); if you are installing the new version from scratch, it will use the new directory structure (areas stored in My Documents). In WEAP, go to Help, About WEAP to see which directories WEAP is using for program and data.
Stockholm Environment Institute, U.S. Center