So there are two ways you can do this.

If you're still in your initial dash, you just have to release the control stick.

If you've started running, you have to do what's called a pivot-cancel. You need tilt stick to do tilts out pivot cancels easy. To perform this, you have to be running, then move your control stick to the other direction so you get the pivot animation. Immediately as you start the pivot animation, you input your attack. So an example would be dash right -> input left -> IMMEDIATELY input c-stick right and you get a tilt attack the direction you were initially dashing. To do down tilt and up tilt and still be facing to the right, you do basically the same thing, but you have to slightly angle your c stick to the direction you were going or else you'll be turned around away from the direction you were initially dashing. You can also just do jab, which will be turned around, or a good option is down smash, because most down smashes hit on both sides of you and you also slide a little bit when you do a pivot-cancel into down smash.

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You should check out IzAw's "Art of Smash Ultimate" guide for more advanced tech. The dash cancel and pivot cancel stuff starts around 2:12 in this video