I'm trying to change the client_max_body_size value, so my nginx ingress will not return 413 error.

I've tested few solutions.
Here is my test config map:

kind: ConfigMap
apiVersion: v1
data:
  proxy-connect-timeout: "15"
  proxy-read-timeout: "600"
  proxy-send-timeout: "600"
  proxy-body-size: "8m"
  hsts-include-subdomains: "false"
  body-size: "64m"
  server-name-hash-bucket-size: "256"
  client-max-body-size: "50m"
metadata:
  name: nginx-configuration
  namespace: ingress-nginx
  labels:
    app: ingress-nginx

These changes has no effect at all, after loading it, in the nginx controller log I can see the information about reloading config map, but the values in nginx.conf are the same:

root@nginx-ingress-controller-95db685f5-b5s6s:/# cat /etc/nginx/nginx.conf | grep client_max                                                                                                       
                        client_max_body_size                    "8m";
                        client_max_body_size                    "1m";
                        client_max_body_size                    "1m";
                        client_max_body_size                    "1m";
                        client_max_body_size                    "1m";
                        client_max_body_size                    "1m";
                        client_max_body_size                    "1m";
                        client_max_body_size                    "1m";
                        client_max_body_size                    "1m";
                        client_max_body_size                    "1m";
                        client_max_body_size                    "1m";
                        client_max_body_size                    "1m";
                        client_max_body_size                    "1m";

My nginx-controller config uses this image: quay.io/kubernetes-ingress-controller/nginx-ingress-controller:0.13.0

How can I force the nginx to change the value? I need to change it globally, for all my ingresses.

You can use the annotation nginx.ingress.kubernetes.io/proxy-body-size to set the max-body-size option right in your Ingress object instead of changing a base ConfigMap.

Here is the example of usage:

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: my-app
  annotations:
    nginx.ingress.kubernetes.io/proxy-body-size: "50m"
...