Fastly - how to change origin timeouts?

When an origin does not respond in a timely manner or generally misbehaves, Fastly will send a 5xx error back to the user. 
If this behavior is systematic and there is nothing to do on the origin server side, there are 2 things to consider on Fastly: 

1. Increase the time out on the origin server. See this article for an explanation.

2. Serve a designed 5xx error page. Best is to use the Fastly synthetic response option conditioned by the beresp.status variable.