Team Build scripts have this nice comment section on how to edit the script, however this does not work...
Normally you will not be able to edit a build script because the Source Control Explorer (SCE) disables several menu options you need.
This is easily solved by creating a mapping from source control's build folder to a local folder using the command line interface to Team Foundation Source Control.
So open a Visual Studio Command Prompt, and move to your project's folder (you need to do this so h.exe can find the team project from your current directory).
Then open the "Edit Workspace <your workspace>" dialog using:
h workspace
In the working folders list scroll down and click on an empty line to create a new working folder:
Use the ellipsis button in the Repository Folder column to browse to the $/<Your Project here>/TeamBuildConfigurations folder in Team Foundation Source control.
Then use the ellipsis button in the Local Folder column to browse to a local folder on your hard disk. Create a new "TeamBuildConfigurations" folder beneath your project's folder and select that one.
Hit Ok to complete a new workfolder mapping.
Now go back to SCE and hit refresh. This should enable the "Get Latest Version" menu option on the TeamBuildConfigurations folder (you might need to close the window to force a full refresh). Perform a get latest version to get the build configuration files locally.
Now you can edit!