Easiest way to be sure is to restart Resolve.
In my testing on macOS - and Windows could be slightly different, I don't know - it will update automatically eventually, but it's not 100% reliable. I'm not really sure what triggers it exactly.
Here's a demonstration video showing me in Photoshop and Resolve:
You can see how it's quite unpredictable. You can also see me attempting to clear the Fusion cache and re-render the current frame. That would be important if the node was already cached (you can tell that by the green bar along the timeline in the middle of the screen), but it won't in itself cause it to detect PSD changes.
You can also see me going back to the Edit page to look in the Media Pool and try viewing the clip there, which sometimes works and sometimes doesn't.
Well, sometimes going to Edit shows the updated image. Whether it was actually going to Edit and opening the file in the media pool that triggered the update, I don't know. It could be updating on a timer in a background or something.