How to come back again & modify existing records/ update other subpages?

They can come back again & modify existing records/ update other subpages whenever they want.

If the form needs to be saved with the information entered, you'll need to enable Auto-fill feature on the form. This feature helps to save the filled information in the browser storage so that the user can come back and retrieve the information again by accessing the Form URL. However, since the information is stored in the browser storage it is required to access the form using the same system and browser. You may also consider using session save method to access the filled form from any other system and browser.

If you are referring to edit the record after submitting the form, you may consider placing an edit link in the notification email so that the user can come back later by clicking on the link received by them after submitting the form.

Hope this information helps!