How does one unlearn being offended?

Being offended is judging another. Judging doesn’t help your inner self. It only brings you down. Some things are truly abhorrent but most things must be ignored as they don’t really affect you personally. Consider also that everyone has a different life experience and views and if their views don’t really affect you in any tangible way then their speech or acts are irrelevant in your life and should be forgotten.

Depend on who’s doing the offending

Willpower. Offence can ONLY be taken.

I'm not sure. It happened to me via an epiphany of sorts. A sudden realisation that being offended was a reaction on my part and therefore a choice. It doesn't matter what a person says to me, they can't make me be offended.