iRule to Rewrite User Agent Header

You have another approach, using the switch statement:

when HTTP_REQUEST { 
     Rewrite the User-Agent header value to show up supported browser 
    switch -glob [string toupper [HTTP::header User-Agent]] {
        "*MSIE 6*" -
        "*MSIE 7*" -
        "*MSIE 8*" -
        "*MSIE 9*" -
        "*MSIE 10*" {
             Replace the User-Agent header with supported user agent
            HTTP::header replace "User-Agent" "Mozilla/5.0 (Windows NT 10.0; WOW64; Trident/7.0; rv:11.0) like Gecko" 
        }
        default {
        }
    }
}

A better approach is not using iRule but using a traffic policy (Local Traffic ›› Policies)