Can I change my movement/action in the middle of the turn after an enemy's reaction?

Question

Imagine I'm a rogue and I see an opponent 50' away. In my turn I want to use my movement and my Dash bonus action (Cunning Action) to approach it and use my Action to attack.
Then, after moving just 10' (or more, I guess it may change the answer), another opponent had readied its action so as a reaction it attacks me, and hits me so hard I don't want to go hit that opponent anymore.

• Can I just stop my current movement and move elsewhere/back?
• Do I have to finish my movement and then use the Dash to move away? If that's the case, I assume it depends on how much I've moved to know what can I do.
• Does changing this imply some sort of penalty?

This is just an example, but the question isn't referred exclusively to movement. It could be attacking and then using the bonus action to Dash instead of hitting again. As noted by the chosen answer, it was related to "declaring and acting" and "changing what I declared"

Answer 1

Yes

D&D 5th edition does not require you declare what you are going to do ahead of time. I don't think there's any specific source to determine this in the Player's Handbook itself, because it's sort of asking to prove a negative. But this answer to a previous question describes quite well how there is no stage at which you declare what you will do with your turn. Simply, when your turn comes up in the initiative, you take your turn, and you can break up the various options you might have available in any order you choose.

This Tweet by Jeremy Crawford (lead designer of D&D) further helps clarify things:

D&D combat doesn't have an action-declaration phase. Things happen in order, and you can be interrupted at any moment by a reaction, trap, or the like. You can say, "I plan to take the Attack action," but that has no rules relevance until you're actually taking the action.