You can create or discover a situation aspect using the create an advantage action (page XX). You may also create boosts that way, or as a result of a tie or success with style when you overcome an obstacle, attack, or defend.

You can remove an aspect provided you can think of a way your character could do so—blast the Raging Fire with a fire extinguisher, use evasive maneuvers to escape the pursuing guard that’s On Your Tail. Depending on the situation, that might require an overcome action (page XX); in this case, an opponent could use a defend action to try to preserve the aspect, if they can describe how they do so.

However, if there’s no narrative block to removing an aspect, you can simply do so. If you’re All Tied Up and then a friend unties you, the aspect goes away. If there’s nothing stopping you, there’s no need to roll.