extending OPT to 29 months relies on  your major.  If your major falls on those categories, you just need to fill a form like you did while applying for your initial OPT. Since your H1 is already in progress, I am not sure if you can do it now . The best thing would be me contacting your International advisor.

If my H1 was under processing, I wouldnot have worried about this ;)

Even this reply is not a solution, it definitely gonna birng your thread on top for a while. Expecting some one to come up with good solution.