Here's the SQL for an Update Query. Just copy and paste it into the SQL View of the query design in Access and then change MyTable and MyField to the relevant names. Save it and then run the query.
Code:
UPDATE [MyTable] SET [MyField] = StrConv([MyField], 1);
is the text that you enter that needs to be uppercase of a standard length? i.e. will the length of the data entered ever change sizes or go beyond a certain length.. ?
You should only need to run the query once, from then on you should be ensure the string is in upper case before you insert the data into the table by wrapping the string in strconv()/Ucase() within your form etc.