Well, this depends on squid version.
For 2.5-3.1 you can use just header_replace:
Code:
header_replace User-Agent Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Win64; x64; Trident/5.0)
For recent 3.1+ you should change the above, to:
Code:
request_header_access User-Agent deny all request_header_replace User-Agent Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Win64; x64; Trident/5.0)
Regards