At the risk of being simplistic and reductive... you just... stop.

Unless it's calling or texting, what you do via phone can almost always be done on a computer/laptop. Allot a certain time of your day/week for handling life shit via a desktop or laptop, and a certain time of the day/week for reading news. Limit it to just that timeframe, and try to keep that timeframe small. Avoid social media as much as you can within that.

Honestly, the only way I found is to go cold turkey and really focus on hating the smartphone and "always online" culture. Reading up on the dangerous effects this stuff has on us and our children really helps with motivation, too. You won't stick to it if it's just for a specific reason like improving sleep or trying to read more books or whatever