The first thing to try is just flexing the blade opposite way to the bend. You may have to flex it right back on itself - a 180 degree bend - and see if that corrects things. Sometimes, this works cold, sometimes not; depends on the exact grade of steel the blade is made of.
If that doesn't work, try pouring plenty of boiling water over the blade - get it good and hot - and flex it again. You might need two or three goes - the blade loses the heat quite quickly. You'll need a decent pair of thick leather gloves (or similar hand protection) for this.
Another method (probably the professional one!) is a bit more involved. Using the polished cross-pein of a smallish Warrington pattern hammer (about 8 to 12oz), and holding the blade firmly hollow side down to a piece of fairly solid hardwood, strike a line of blows from toothline to back, so that the hammer pein makes a line across the saw blade. Repeat at about two-inch intervals along the saw. Start with very light blows, check the effect, and increase the weight of blow if necessary. Check progress frequently. If necessary, strike more lines a bit closer together. Thicker sawblades will need heavier blows, with lines of blows closer together.
The first method is probably the best; the second ideally needs a bit of practice on a scrap saw first, just to build up the confidence, and the feel for it.
The final (pragmatic) option is just to buy another saw with a straight blade!
