You can't change this within Funnelback and is not recommended because you might break other functionality (e.g. s is another valid CGI parameter in Funnelback that is designed for system based queries).
However if you must change it you will need to put something in front of Funnelback that rewrites the URLs and maps the input query string to Funnelback's correct parameters. You can do this with something like Apache's rewrite rules, nginx or open resty. However, you will need to be careful to pass through all parameters you don't map and there is a risk that you may miss some mappings, lose access to some functionality or in the worst case break Funnelback completely because required parameters are no longer available..
