Modify HTML content with an iRule

A couple of things you can do.

If it's only a single static value you're trying to replace, I believe you can do in in a stream profile.

Otherwise, you can add an empty stream profile to you VIP and then perform the replacement(s) in an iRule using STREAM::expression. You should look at STREAM too to get familiar with how it'll work.

Always make sure you remove the Accept-Encoding header too so the response won't be compressed (if it is compressed, the rewrite won't work properly).

Here's a good example from the STREAM::expression wiki page:

     Example which replaces http:// with https:// in response content
     Prevents server compression in responses
    when HTTP_REQUEST {

        Disable the stream filter for all requests
       STREAM::disable

        LTM does not uncompress response content, so if the server has compression enabled
        and it cannot be disabled on the server, we can prevent the server from 
        sending a compressed response by removing the compression offerings from the client
       HTTP::header remove "Accept-Encoding"
    }
    when HTTP_RESPONSE {

        Check if response type is text
       if {[HTTP::header value Content-Type] contains "text"}{

           Replace http:// with https://
          STREAM::expression {@http://@https://@}

           Enable the stream filter for this response only
          STREAM::enable
       }
    }