Login to Back Office > from the left menu go to ‘Settings’ > click on ‘Shop Settings’ in the submenu of ‘Settings’ > select shop from ‘Select Store’ option > switch on ‘
’ (i.e. On) once for ‘reset item stock‘ 
Pop-up will come for confirmation, click on ‘YES’
‘Shop Item Stock reset Successfully’ Pop-up will come > as the last step click on ‘Save Settings’.