How can i transform the data entries by a user in lowercase to uppercase.

If you just need it displayed as uppercase in a form or report, put a > in the format property of the field. If you need the data actually changed to uppercase, I don't know.

Thanks for the answer I can use it further, but in this particulary case I need the data to be in uppercase in my tables.
If it's not possible, how can i require the user to entry the data in uppercase ?

In the AfterUpdate event of the field in question use the UCase() function.

Me.YourField = UCase(Me.txtYourField)

The above statement assumes the field name is YourField and the control name is txtYourField. Change the statement accordingly. If the controlsource and the control name are currently the same value, change the control name by adding a prefix such as "txt" so that Access will be able to figure out what object you are referring to.

Thanks for the answer, it works well.