Fedoras usually have a smooth surface – they don’t have much ‘nap’ to the felt – so you can steam press them without ruining the finish. Get a damp cloth, place over the bit of hat that needs reshaping, and apply a hot iron to force steam into the felt. The felt will become pliable/bendable and you can shape it gently by hand.

If it’s an older hat it may have gone a bit floppy and won’t keep shape well. I’ve used spray starch (follow directions on can, but put a cloth over the area before you iron lightly) to add a bit of stiffness. A proper spray on felt stiffener (from a millinery supplies company) should work better.
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