I dont know why but my young brother laptop (Dell) keeps turning the FN button back to default, I mean, the buttons fn+F1 fn+F2 fn+F3 (mute, volume down and volume up). turn volume just by pressing them, not the FN button. It used to work the vice versa. I changed it many times in bios, but it keeps changing everytime..

how to solve this?

I wish you had the exact model but the last time I saw this, the setting was in the BIOS.

On my Dell laptop I can toggle the top row between function keys and media keys by pressing Fn+ESC. Pressing Fn+F1 toggles the mute and occasionally I miss F1 and press esc by mistake (thereby toggling the function keys). Perhaps this is what is happening with you.