What are some ways I can stretch these boots without ruining them?

I have just received a new pair of Doc Martens today. I have already returned one pair because they were too tight not too small, necessarily, but they pinched where the tongue was sewed into the boot and this new pair is a little tight. I don’t want to return them because, quite frankly, returning them once and ordering them again was enough of a hassle, but also because I ordered them from my own store and as they were very pricey, a return would hurt the day’s profits. Strange as this may sound, this could conceivably lead to cutting of hours during the week if enough money is not made. The boots are not leather, if that would make a difference. I would just very much like to keep these boots but if I cannot reasonably stretch them, I will return them and get the next size up.