That's a customizing thing, more than anything... You could boil the water and it'd be fine. Hot would probably do the trick though, even under a hot tap.
Plastics that are flexible (PVC's) like on most Hasbro items these days (basic figures and such) are very pliable when heated in water, and they tend to take their "memory" back to their original molded shape when heated like that. There's a technique in customizing called the "boil & pop" to pull figures apart, and also the "boil & freeze" which lets you slightly repose say an ankle. I reposed POTF2 Ceremony Luke for instance, as his ankles were molded at an odd angle, and so he tended to fall over. After a boil & freeze, his stance was flat and he stood quite well.
What I'd suggest, run the tap water in your bathroom sink (be careful if you've got something small you're doing this with, as you don't wanna lose anything down a drain. Use a noodle strainer if you've got one maybe) and see if the items take their natural shape on their own running them under that hot water. Help them along (be careful not to burn yourself. Use dish washing gloves or something) by maybe helping to gently bend them back into shape. Sometimes they'll just retake shape right away though. Part of me guesses that is what would happen with the Miniatures.