Hi Mohammed,
According to your description, my understanding is that you want to change the Date only field to Date Time in existing fields in SharePoint.
I recommend to edit the date column in the browser directly.
Please go to the List settings in the list where the content type is used and then click the date field under Columns. After that you can edit the date field to Date and Time format and all the existing fields will be updated with 12:00 AM value with the
date value.
Best regards.
Thanks
Victoria Xia
TechNet Community Support
