Need Help in iRule to Modify HTTP header parameter.

HTTP headers are not case sensitve in correctly implemented HTTP servers (see https://www.rfc-editor.org/rfc/rfc9110.html#name-field-names) , so the BIG-IP iRule command to handle them is also case insensitive (https://clouddocs.f5.com/api/irules/HTTP__header.html).

You'll have to first grab the header names (with any case), delete the headers (with any case), then re-add new headers (with your desired case). Something like this should work:

when HTTP_REQUEST {
  set sname [HTTP::header value sname]
  set uaccount [HTTP::header value uaccount]

  HTTP::header remove sname
  HTTP::header remove uaccount

  HTTP::header replace SName $sname
  HTTP::header replace UAccount $uaccount
}