I have straightened tenon saws by holding part of the saw plate in the bench vise and bending the rest of the saw in the opposite direction to the bend. It’s not a very elegant way of doing it and I’ve never gotten a saw perfectly straight that way but they’ve always ended up better. Might be worth a try but be very, very careful. I’ve only removed small bends – never a huge one. Might be worth checking the original listing to see if it said the saw was bent. If it didn’t, check if you can return it.
Often and quite separately to backless hand saws, you’ll find a tenon saw with a rippled/kinked saw plate where it has slipped within the steel or brass back.
Beware hammering saw plates, as this can reduce the tension in a saw plate if you’re not careful.
Before doing anything I’d check the saw back to ensure the kink isn’t due to a bent spine. If you find the spine is bent it will bend/kink the plate, but this can be straightened in a vice.
One quick fix – which often works – simply involves gently tapping the spine of brass/steel back along it’s full length. This usually springs the plate back into place, but – if that fails – you can grip the tip of the saw plate in a vise before gently rapping the tip of the steel/brass spine. A drop or two of lock-tite will hold the plate in position if you find the plate slips again.
Only hammer the saw plate if either of the above two methods don’t fix the kink.