First, make sure that your phone's volume down button isn't sticking. Sometimes, a bit of dust/debris or something sticky might cause the volume down button to stick and keep the phone in safe mode.
If a stuck volume down button turns out to not be the issue, then try this:
1. First turn off the phone.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe Mode: OFF" or something similar.
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