The strap button itself should come off the jack if it is this model:
http://www.allparts.com/EP-4161-001-Switchcraft-Nickel-End-Pin-Jack_p_1379.html
If you can get the strap button off, there should be another nut and washer underneath it that you have to first loosen, take the jack out through the soundhole, adjust the nut on the interior side of the jack, and then put it back in the endblock. Make sure when you are trying to loosen the strap button that the rest of the input jack is not spinning. IF it does, you may damage the wires and have to resolder it as well.
I usually reach into the guitar with a #2 Phillips screwdriver through the input jack hole. Since you can't reach the endblock with just your hand, by putting the screwdriver in the input jack hole, you can push the jack onto the screwdriver and it should hold well enough to pull it back out with the jack attached. Install the washer then nut, there should be a hole going sideways through the jack as well. You can put an Allan key through that to hold it in place while you tighten with a thumb wrench.
For a diagram:
http://www.lrbaggs.com/sites/default/files/pdf/element_active_manual.pdf
Basically, you're crushing the wood between 2 nuts and washers, that's all that keeps them in place most times.