making changes to cookie

Yes, the easiest way is to use an irule that removes any un-wanted cookies.

when HTTP_REQUEST {
    HTTP::cookie remove 
}

Absolutely, and it's easy enough with iRules provided you know which cookies should be removed:

when HTTP_REQUEST {
   HTTP::cookie remove 
}

If you don't know exactly what cookies you want to remove but instead know what cookies you want to keep it's a bit more complicated, because then you need to loop through all the cookies and check if it's an allowed cookie or not. If this is the case I can make an example for you if you want.

Thanks for the responses, is it possible to get the value of a single cookie with name say "test-cookie" from the list of cookies and replace the entire cookie header with just this one? Henrik can you point me to such an irule please?

Sure, it would look something like this, might be typos since I'm doing this from the top of my head:

when HTTP_REQUEST {
   foreach c_name [HTTP::cookie names] {
      if { !( [HTTP::cookie $c_name] eq "test-cookie" ) }{
         HTTP::cookie remove $c_name
      }
   }
}

This should remove all cookies except for the "test-cookie" you want.

when HTTP_REQUEST {
    set sessionCookies [ HTTP::cookie names ]
    foreach mycookie $sessionCookies {
        if { not ($mycookie equals "test-cookie") }{
            HTTP::cookie remove $mycookie
        }
    }
}

This should remove all cookies except for the "test-cookie" you want.

when HTTP_REQUEST {
    set sessionCookies [ HTTP::cookie names ]
    foreach mycookie $sessionCookies {
        if { not ($mycookie equals "test-cookie") }{
            HTTP::cookie remove $mycookie
        }
    }
}

Thanks a lot, I will try these out. Is there an article on devcentral which points to various things that can be done with a HTTP::cookie like getting the names, rewriting, removing etc., ?