Change and reset child drop-down items based on parent drop-down item change

Hello good day,
I’ve a requirement to populate two drop downs which data will be fetched from APIS for both the drop downs and I’ve to change the child drop-down items based on parent drop-down item selected value changed.
I’m able to populate both the drop-down successfully
I’m able to change the child drop-down items based on parent selected value
Issue is I’m not able to set the default value for child drop-down when parent driven changed, I mean whenever parent drop-down item changes I’ve to set the child drop-down default item as Select Option.
Help me here
Jyothi.