how wide 28" can be ? or how to beat mountain trails with 28"
Ok so I presume your tyres are 700x40C. First of all, that pressure sounds high - is it recommended or max pressure? I am sure you could decrease it a fair bit, although if you decrease it excessively you run the risk of pinch puntures.

If you do want to change your tyres, 700c road rims and 29" MTB rims are the same diameter, so you can use, for example, a 29x2.0" tyre - if your rim is a suitable width - see the table at the bottom of this page for recommended rim/tyre width combinations

EDIT: As armb has quite rightly pointed out, you will also need to check for sufficient frame clearance if fitting wider tyres - and remember you'll need to allow some space for mud accumulation if riding off-road