I didn't see a manual online in searches.
It might be able to discover something from questions on the web sites sellling it, or ask your specific question and see if it is answered.
Quoted from llink above:
How to use FM Radio in this device
This question is from Bluetooth Shower Speaker with FM Radio and Carabiner in Black
November 28, 2015
How do I use the FM Radio in this device. The device description says FM Radio but there is no mention of how to use it.
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December 29, 2015
If you push the middle button it will switch from Bluetooth mode to radio. The first time I used mine it immediately searched through all the radio stations it could detect and store that information. Kind of like when you plug in a new TV. Once that scan is done it will play the lowest numbered station, you then use the buttons on the left or right to advance to the station you want. Don't press middle button again or it will go to Bluetooth. Good Luck
December 5, 2015
Turn on the device with the power button and then press and hold the "play/pause" button down until you hear the radio. Press and hold the back and forward button to control the volume and press the back and forward button with a quick release to switch the radio channels.
December 28, 2016
Hold down the play/pause button until the radio turns on. First time you use it, it takes a couple of minutes to program the channels then use arrows keys to go through the programmed channels.