Storyteller fiat and experience point expenditure.
On page 642 of the 20th Anniversary Edition Core rules for Mage: The Ascension, it reads:
Generally, Merits and Flaws cannot be purchased after character
creation, although the Storyteller may decide to give them to
characters in place of experience points if those Traits seem
appropriate to the events within the chronicle. Depending upon those
events, a player might be able to use experience points to buy off a
troublesome Flaw. In all cases, Merits and Flaws are optional Traits.
The Storyteller may disallow some, all, or none of them – it’s her
choice.
And then, on page 645, we find:
Flaws offer extra freebie points during character creation. Even so,
they’re not only point-magnets; each Flaw presents a story-based
challenge that must be faced and overcome throughout a character’s
life. Until and unless that Flaw gets bought off with experience
points and some significant events within the chronicle, a Flaw
remains a monumental pain in the ass for a mage, his companions, and
the player who decides to take it.
So, you need the ST's permission, an in-fiction trigger, and a spend of XP. How many XP? That question isn't answered in the experience point tables in M20; check with your storyteller.