The client should switch to a different protocol such as TLS/1.0.
What does HTTP 426 mean?
The 426 Upgrade Required code is part of the 4XX class. This is a client error code — the request was faulty, unauthorized, or cannot be fulfilled. The problem is on the request side.
When is it used?
A server returns 426 Upgrade Required in situations matching the definition above. You will encounter it inspecting network requests, reading logs, or testing an API. Whether it signals a problem depends on its class: 2xx and 3xx are normal, while 4xx and 5xx usually need attention on the client or server respectively.