How do I change query param name inside the HTTP::query object in my iRule?

If you just want to statically set HTTP::query as some arbitrary value, you can use something like this:

when HTTP_REQUEST {
 HTTP::query "somethinglikethis"
}

If I may add, you can extend that to test for specific param names. Example:

when HTTP_REQUEST {
    if { [string tolower [HTTP::query]] contains "qc=" } {
        HTTP::query "qd=[URI::query ?[HTTP::query] qc]"
    }
}

You could also change it with a string map:

when HTTP_REQUEST {
    if { string tolower [HTTP::query]] contains "qc=" } {
        HTTP::query [string map -nocase {"qc=" "qd="} [HTTP::query]]
    }
}

Thanks everyone!

I tried the suggestions, and somehow I feel that the HTTP::query is read only, unable to modify it. What worked though is resetting the HTTP::uri object as -

HTTP::uri "[HTTP::path]?$query"

, where variable query holds modified HTTP:query string.

I feel that the HTTP::query is read only, unable to modify it.

the write functionality is introduced in 11.5.0. :)

ID385615 - RFE: Make HTTP::query iRule command support writing data