How do I stop Preston Garvey from being hostile to me?

I pickpocketed someone in the group and now he is trying to kill me. He won't die. How do I make him non-hostile?

When you get someone (or even an entire settlement/town) angry at you, you need to give him some time to cool off.

Get away from that location and sleep/wait in a safe spot for at least 48 hours.

Once you've been away for at least two in-game days you can come back safely. He will have forgotten everything about your offenses by then and be friendly once again.

If you do not mind a little lost game time you can use the "load" function to go back to a time before you pickpocketed him. So just load an earlier save. This also works well for times when you fall into a hole or water where you cannot get out or just feel you make a mistake that you would like to correct.