Changing Data In A Field Using A Form

I have made attempts at this but none have worked, so I am looking for guidence.

What I creating is a database that I can track students and their community service hours.

I have made several versions but I figured the experts could point me in the right direction.

So far I created two tables:

tblStudents
STUDENT_NAME - Text
HOURS_GIVEN - Long Integer

tblHours
STUDENT_NAME - text Links to tblStudent
DATE_COMPLETED - Date
HOURS_COMPLETED - Long Integer

I created a form "frmStudent" with the following:

a drop box to chose a student name from "tblStudent"
a text box "txtSTUDENT_NAME"
another text box "txtHOURS_GIVEN"
another text box "HOURS_REMAINING"

Then I have a subform that has two text boxes "txtDATE_COMPLETED and "txtHOURS_COMPLETED"

I need a code that will enable me to chose a student from the combo box and then have that students name and hours given automatically enter into the respective text boxes in the main form. Then I would enter the date and hours completed into the respective text boxes of the subform, which in turn would update the fields in "tblHours" at the same time subtracting the new hours from the hours given and displaying the new total hours remaining in the respective in "txtHOURS_REMAINING" of the main form. Can anyone give me any help in coding that? Thanks in advance.