Enterprise Tester requires version 4.5 of the .Net Framework to be installed for version 4.9 and above.  If you are using either of the executable installation files (.exe), the installer will attempt to install this for you.  Your ET server must have internet access as the installer will try to download the .Net Framework 4.5 installation file. 

If you are using the Zip File Installation to manually upgrade or installing Enterprise Tester, then you must manually install the .Net Framework 4.5 in advance.

When installing the .Net Framework 4.5 you have two options - either installing from a "web download" (a small download, which will then install only the components you require) or you can download a "redistributable" which is a large file containing all the files necessary for the .Net Framework 4.5 to be installed on any machine.

If you plan on installing the Framework on more than one machine within your organization it is suggested that you use the redistributable, to avoid multiple downloads.

The web downloader can be downloaded from:

http://www.microsoft.com/en-us/download/details.aspx?id=30653

It is approximately 1mb in size.

The redistributable can be downloaded from:

http://download.microsoft.com/download/2/7/3/273CB528-CAB3-4E8D-88D2-4477F0A1DAE4/enu_vs_common/netfx_ult/dotnetfx45_full_x86_x64.exe

It is approximately 50mb in size.