To cycle through the saved radio station frequencies, press the Play/Pause/M+ button on the remote or on the HM3817DTBLK. On the remote, this is the middle button in the third row (between the tune/skip/search - and tune/skip/search + buttons). On the music system, this is the third button to the right of the LCD display.
This is described in the manual which is available from the product page: https://gpx.com/in-home/home-music-systems/hm3817dtblk.html
. Scroll down and click on Downloads. The manual is the first link. The direct link to the manual is https://gpx.com/amfilerating/file/download/file_id/49096/
. See page 4-5 (page 2 of the PDF).
I hope this helps.