In each of the following directories:

C:\Program Files\Blackbaud\bbappfx\vroot
C:\Program Files\Blackbaud\NetCommunity
C:\Program Files\Blackbaud\PluginService
  1. Edit the web.config file with a text editor
  2. Locate and replace the following text: <add key="ConnectionString" value="Server=TEST_SERVER ;user id=ID_HERE;password=PW_HERE;database=TEST_DB" />
  3. Note: Be sure to replace the "filler" text with valid server authentication and connection information
  4. Save each file
  5. Restart IIS
  6. Open the NetCommunity Web Server, check the test config page: //localhost/testconfig.aspx

For more information please see the following resources:

How do we migrate Blackbaud NetCommunity to a new server?