Why not just redirect to HTTPS with 301?

301 to HTTPS is the typical fix. 426 is more specific but less commonly used. For HTTPS upgrades, 301 or 308 is what most servers actually do.

426 Upgrade Required: HTTP Status Code Meaning, Causes, and Fixes

What 426 Upgrade Required means

426 Upgrade Required tells the client that this resource requires a protocol the current request did not use. Most commonly seen when an HTTP endpoint requires HTTPS, or when an HTTP/1.1 endpoint requires HTTP/2 or WebSocket. The response must include an Upgrade header listing acceptable protocols.

When servers should return it: Return 426 when a resource requires a specific protocol not used by the current request.

Common causes

HTTP request to an HTTPS-only endpoint
HTTP/1.1 request to an HTTP/2-required endpoint
WebSocket required but client did not upgrade

How to fix 426 Upgrade Required

Use HTTPS instead of HTTP
Upgrade HTTP client to support HTTP/2
For WebSocket, send Upgrade and Connection headers

Example response

HTTP/1.1 426
upgrade: TLS/1.2, HTTP/2
connection: Upgrade
content-type: text/plain
This resource requires HTTPS

More references

For a one-page reference of all HTTP status codes, see the HTTP cheat sheet. For testing API responses, try the API Tester tool. For inspecting responses on the command line, the curl cheat sheet covers the most common flags.