To reset the JBL Charge 5, Charge Essential or Charge Essential 2: 

Volume+ & Play/Pause Long press for 2 seconds while the speaker is ON

Please note: If the Charge 5 was previously paired with any phones/tablets, please remove the speaker from your list of paired Bluetooth devices. Now it has been restored to its factory settings, it will automatically enter into pairing mode the first time you power it on.