How? Some people inch out. Some leap. Some go flying out only when given a good shove. And some just work on widening the boundaries of their comfort zone a little at a time.
There are a lot of ways to widen your boundaries safely and comfortably, and you can find them if you look for them. Press yourself to edge out in small, undramatic ways. Do it when you don’t have to. Do it when nothing is at stake. Do it when the threat level is low. Practice and gain muscle strength.
For example, if you dread giving presentations at work, don’t start with 40 people in a conference room and 80 more on dial-in. Start with four friendly people in the smallest conference room you can find.
If you don’t like trying to begin a conversation with an interesting stranger at a party, practice by making a casual remark to someone while waiting for an elevator or a printout, exactly as mentioned here.
Confidence breeds confidence.