Removing or changing an aspect

Game and character aspects change through advancement. See the Milestones section in The Long Game for that.
If you want to get rid of a situation aspect , you can do it in one of two ways: roll an overcome action specifically for the purpose of getting rid of the aspect, or roll some other kind of action that would make the aspect make no sense if you succeed. (For example, if you’re Grappled, you could try to sprint away. If you succeed, it wouldn’t make sense for you to be Grappled anymore, so you’d also get rid of that aspect.)
If a character can interfere with your action, they get to roll active Opposition against you as per normal. Otherwise, GMs, it’s your job to set passive Opposition or just allow the player to get rid of the aspect without a roll , if there’s nothing risky or interesting in the way. Finally, if at any point it simply makes no sense for a situation aspect to be in play, get rid of it.