According to the deCONZ documentation: 1 = most sensitive, 21 = least sensitive.
You don’t have to restart anything, but because it’s a sleeping device you have to wake up the device (and keep it awake) if you want to apply the setting immediately (if the device is kept sleeping, deCONZ will queue the configuration change until the device wakes up by itself, but you won’t know exactly when that is). Short-press its button to wake it, and short-press it once every few seconds to keep it awake.