You could use the On Load event or the On Current Event.

Thanks Adam, it only updated the first record on top? how do i update all the record?

Oh yea, that is going to be more advanced. Probably the easiest way would be a recordset, loop through all your records and apply the formula. That might be beyond your VBA knowledge now, I believe recordsets are first covered in Access Developer 16.

Basically would be something like
dim rs as recordset
rs = currentdb.openrecordset("Select * From Table where Enter your Crtieria")
while not rs.eof
'whatever formula you want to apply to the fields, like rs!DateField=Date()
rs.movenext
wend
rs.close
set rs = nothing

You could just use a simple RunSql statement. No need for a recordset.

Recordset is #1 Rick.

In fact, I think we'd all be better off if we just wrote a recordset for every single field.

rs = Currentdb.openrecordset("Select * from customerT where CustomerID=1")
while not rs.eof
rs.Edit
rs!Name=InputBox("Enter Value")
rs.update
rs.movenext
wend
rs.close
set rs=nothing

on EVERY field, there's no such thing as going to far right...
We don't just do
Name="John" here, too easy