Are you sure you try to in the C.Fn section? Here is from the manual: " If C.Fn II-3 [hignlight tone priority] is set to [1: Enable}, the settable ISO speed range will be ISO 200-1600. (p164)" "With C. Fn I-3 [Set ISO speed range], the ISO speed range can be extended to ISO 50(L) to 3200 (H). (p158) "
The manual is always your friend.