EC2 Instance Key Pair Change

We have Ec2 instance which is connected to Elastic Beanstalk. The instance has an Auto Scaling group connected which has a Launch Configuration setup. Now we do not have access to the existing keypair of the instance, hence we are trying to change the keypair for the instance. To achieve this we have created a new launch configuration with a new keypair and connected that launch configuration to the existing Auto Scaling group. But the changed keypair is not reflecting on the instance. That is where we are stuck with and we need a way to change this key par so that we can access the instance.