Please note that this article refers to some older versions of plesk than what are currently available and we strongly recommend updating your Plesk install to protect against misuse.
In order to be able to change the asp.net versions of domains through the Plesk control panel you will first need to install the "pleskpp_126.96.36.199_20070920_x64" template.
This is done in main customer control panel under:
Parallels Plesk Panel >> Applications >> Install
In the list of templates select the "pleskpp_188.8.131.52_20070920_x64" template and then press "Submit" to start installing it.
Once this is done you should be able to select asp.net version for your websites in the Plesk control panel. If not you might need to contact us and\or try changing the versions by hand: