In Tuck Everlasting, from chapters 23-25, how is the wheel turning again?

The line of text that your question is referring to is found in chapter 25.  

The first week of August was long over. And now, though autumn was still some weeks away, there was a feeling that the year had begun its downward arc, that the wheel was turning again, slowly now, but soon to go faster, turning once more in its changeless sweep of change.

But the wheel that the text is speaking of was first introduced in the opening prologue of Tuck Everlasting.  

The first week of August hangs at the very top of summer, the top of the live-long year, like the highest seat of a Ferris wheel when it pauses in its turning.

The "wheel" is referring to a passage of time.  Specifically the passage of time from season to season.  In the book Tuck Everlasting, author Natalie Babbitt  tells readers that the first week of August is like that momentary pause at the top of a Ferris wheel.  At that point, the rider isn't moving up or down.  He/she is stuck in place for a fraction of an instant.  The reason August is like that is because summer is hanging on for a bit longer before the weather turns and fall begins to creep in.  

Most of Tuck Everlasting takes place during that first week of August.  The text tells readers that it was uncomfortably hot during that week.  Most people, when confronted with hot and humid weather don't feel like doing anything.  Motion makes it worse.  So staying as still as possible is key.  That coincides with August being like the momentarily frozen Ferris wheel.  The weather is stagnant.  The people are stagnant.  Chapter 23 really drives home those feelings.  Time seems to stand still for Winnie, because she is anxious to help Mae escape and the weather makes it even worse.  

It was the longest day: mindlessly hot, unspeakably hot, too hot to move or even think. The countryside, the village of Treegap, the wood—all lay defeated. Nothing stirred.

By chapter 25, several weeks have passed since the night of the rescue.  The hot weather has broken, fall is beginning to creep in, the seasonal wheel is beginning to move again, and Winnie is being forced to move on from the Tuck family as well.  The chapter ends with Winnie making a very physical move away from remembering the immortality that the Tuck family offered her.  She pours out the bottle of spring water and her own wheel of time keeps turning.