This one's pretty easy, if you know where to find the solution.
To illustrate it, I created a four-page InDesign document where I put a big number in text on each page, like so:
Go to the Flyaway menu on the Pagespanel — the "hamburger icon" with the three lines at the upper-right of the panel. Click on it and select the Move Pages... menu command, as shown below:
To illustrate the point, we'll move page 4 to be after page 2. To do that, you'd enter i in the Move Pages: edit box. We'll choose After Page in the Destination: options box, then 2 in the edit box to the right. There are other options available. Then click the OKbutton, as shown below:
The end result? What was page 4 is now moved to the page 3 position. And what was page 3 falls to the page 4 position, as shown below.