I'm afraid I don't know of any built-in method for this, we use this method that takes the querystring and sets parameters

    /// <summary>
    /// Set a parameter value in a query string. If the parameter is not found in the passed in query string,
    /// it is added to the end of the query string
    /// </summary>
    /// <param name="queryString">The query string that is to be manipulated</param>
    /// <param name="paramName">The name of the parameter</param>
    /// <param name="paramValue">The value that the parameter is to be set to</param>
    /// <returns>The query string with the parameter set to the new value.</returns>
    public static string SetParameter(string queryString, string paramName, object paramValue)
    {
        //create the regex
        //match paramname=*
        //string regex = String.Format(@"{0}=[^&]*", paramName);
        string regex = @"([&?]{0,1})" + String.Format(@"({0}=[^&]*)", paramName);

        RegexOptions options = RegexOptions.RightToLeft;
        // Querystring has parameters...
        if (Regex.IsMatch(queryString, regex, options))
        {
            queryString = Regex.Replace(queryString, regex, String.Format("$1{0}={1}", paramName, paramValue));
        }
        else
        {
            // If no querystring just return the Parameter Key/Value
            if (queryString == String.Empty)
            {
                return String.Format("{0}={1}", paramName, paramValue);
            }
            else
            {
                // Append the new parameter key/value to the end of querystring
                queryString = String.Format("{0}&{1}={2}", queryString, paramName, paramValue);
            }
        }
        return queryString;
    }

Obviously you could use the QueryString NameValueCollection property of the URI object to make looking up the values easier, but we wanted to be able to parse any querystring.

Oh and we have this method too that allows you to put in a whole URL string without having to get the querystring out of it

public static string SetParameterInUrl(string url, string paramName, object paramValue)
{
    int queryStringIndex = url.IndexOf("?");
    string path;
    string queryString;
    if (queryStringIndex >= 0 && !url.EndsWith("?"))
    {
        path = url.Substring(0, queryStringIndex);
        queryString = url.Substring(queryStringIndex + 1);
    }
    else
    {
        path = url;
        queryString = string.Empty;
    }
    return path + "?" + SetParameter(queryString, paramName, paramValue);
}