try this....
1.create a new query using the table where the field resides you need updating
2.make the query an update query (from the main menu, go to Query - Update Query)
3.select the field you need to update
in the Update To: section enter the following...
LCase([YourFieldName])
Note, the fieldname will be the same as the field you
selected in Step3.
You could also do this using SQL.
UPDATE TableName SET TableName.YourFieldName = LCase([YourFieldName])
I personally like the first method, but it's up to you what you need to use.
hope this helps.