Possible to automatically select text field data on enter

I have been trolling this forum for a couple weeks and have figured out everything on my form except this issue.

I would like my form to automatically select the "data" that is in a text or numeric field when the field is entered. 

For example:  If I open a form that has been filled out previously I would like each field's data to be selected when I enter the field either by tab or clicking on the field.  This way I can overwrite the current data much easier without having to double click to "select all" or backspace to delete the current field.  It is possible that the data should stay the same so I dont want the data to be deleted on enter just "selected"

Thanks for your help,

Joe