Is it in a position that you can easily work on it?
If not I would pull off the manifold.
Next I would purchase a quality helical thread insert kit. You can get them from
http://www.mcmaster.com/
Follow the ditections and drill out the fasitner to the size indicated, then tap the hole with the insert tap. Then you just have to use the tool to thread in the insert. It will be stronger than it was when it was new.
Take your time, and if you start to screw it up, STOP, and take it to a machine shop. The bill will be cheaper than purchasing a new manifold.