You can do it any number of ways.
Pop a pipe cleaner in the stem, just to make sure the airway stays open. Get a large bowl of cold water ready.
You can light a candle, hold the stem about 3" above it and rotate it (keep it moving!) until it heats up enough to bend. Takes about a minute. Bend to shepe and put in cold water - you have to hold it till it cools off.
This method works really well, but there's a danger of burning the stem if you aren't careful.
IF you want, you can use a pot of boiling water, that will make the stem bendable in about 3 minutes. Bend and cool as per before.
Boiling water will yield you a grey, dull stem that needs polishing, so if you don't have that capability, think about not using hot water.
You can also use a heat gun or hair dryer.