#Replace the forward slash (/) in kubernetes.io/ingress.class with ~1.
#Your command should look like this,
$ kubectl patch ingress mying --type='json' -p='[{"op": "add", "path": "/metadata/annotations/kubernetes.io~1ingress.class", "value":"nginx"}]'
#Reference: RFC 6901 https://www.rfc-editor.org/rfc/rfc6901#section-3