Patch minimum/maximum number of replicas in HPA can be done easily by using kubectl patch
command
Patch minReplicas
For example, minReplicas to 2
kubectl patch hpa my-app -p '{"spec":{"minReplicas":2}}'
Patch maxReplicas
For example, maxReplicas to 4
kubectl patch hpa my-app -p '{"spec":{"maxReplicas":4}}'
To work with multiple HPA
Again, wrap it in basic for loop
for i in `kubectl get hpa -o name`; do kubectl patch $i -p '{"spec":{"maxReplicas":4}}'; done;