Hi,
By using translate, concat, substring functions you can achieve your requirement.
The following will translate the value of field1 as you required.
For example, field1 value = lower case and the out put is = Lower case
Here I have used the following expression as default value in the field2.
concat(translate(substring(../my:field1, 1, 1), "abcdefghijklmnopqrstuvwxyz", "ABCDEFGHIJKLMNOPQRSTUVWXYZ"), substring(../my:field1, 2, string-length(../my:field1)))