To edit a field in a Table, go to the Configure mode for the Table component, select the field to edit, and press the Edit button.
A Field Editor dialog will be shown with the current settings, which you can modify.
Wherever possible the system will preserve any information already entered in records: so changing the datatype from a Text to a Large Text field is quite safe.
If, however, you change from a Large Text to a Text field, some of the contents of the Large Text field may not fit in the Text field, and in this case the contents iwll be trimmed to fit. Similarly, if you change from a Text field to a numeric field you should not expect any data to survive from that field.