After Update
Value From a Form

Thanks Kevin, I have watched these videos but they didn't help me with what I am trying to do so I am looking for more information and may have to take the Developers training course.

The Value From a Form show you what you need.
On that page shows the syntax:
       Fields on a single form: Forms!FormName!FieldName
       Fields on a subform: Forms!ParentFormName!SubFormName.Form!FieldName

Go watch it again and give it a shot.  Use the included DB that you can download to look at the code.

If that still doesn't help - be more specific on what you want to do.  
When I update X I want Y to happen.  Pictures help a lot.

If you aren't sure how:  Snip & Sketch
Use the Upload Images to the right of the reply box to upload the image.

Thanks Scott, here is what I am trying to do, screenshots attached.
I have a Parent Form, frmMaskSetDetails and a Sub Form, Mask Plate Details.
The Parent form has a field called Entry Date which is when the record was first created.
The 2 fields in the sub form, Status and Location, will get updated at a later date
What I want to occur is for the field Entry Date to be updated to the current data/time, each time the fields in the sub form are changed.
I want to create an procedure for the After Update Event to do this
Finally, how do I donate to the Tip Jar?
Thanks

Go to the Form properties of the Subform, got o Events tab select After Update.

Private Sub Form_AfterUpdate()

    Parent!EntryDate = Now()
    
End Sub

This will update EntryDate on the Parent Form to the current date and time after any field in the Subform is changed.

I should also say the field will only be updated when you move off that record.
If you want the update to happen immediately add a Me.Refresh to the After Update event of each field in the Subform.

Thanks Kevin
You have EntryDate in your expression but as the field name is Entry_Date i wrote the expression as follows:
Private Sub Form_AfterUpdate()
Parent!Entry_Date = Now()
End Sub
That said, this did not work and produced a Run-time error '2465' MS cant find field 'Entry_Date' refered to in the expression
The Entry_Date field on the Parent form is a text box who's Control Source is in a different table from the subform, not sure if this has anything to do with it.
The expression makes perfect sense to me so not sure why it gives the error

Could it be related to the Master/Child fields that link the subform to the form?

Probably due to the underscore in your field name. Try enclosing the field name in square brackets.

Parent![Entry_Date] = Now()

I tired that, no luck.
As I am still in the design phase of the database I probably should remove the underscore as its a leftover from when I learnt Access years ago in the UK.
Let me do that and see if that resolves the issue
Thanks

I just had another look at your image. There appears to be a space before the underscore.

That was it, my bad for not checking that.
Thanks for the help and let me know how to donate to the tip jar

Tip jar figured out, many thanks for all the help