There are a couple of ways to get the phone out of Safe Mode.
The first option is to use the Status Bar to turn off Safe Mode. Pull down (swipe) the status bar of your Samsung Galaxy S6. Now tap the "Safe Mode" button. This should turn Safe Mode off. Now your phone will automatically restart, so that it boots up out of Safe Mode.
Your second option is to restart your phone.This will often fix the problem on it's own.
However, if the Safe Mode is still running after you restart your phone, I would then check to make sure your Volume Down button is not stuck. Check to see if it has anything stuck in it as well, dust, etc. Anything like this can cause the phone to go into, and get stuck in, Safe Mode.