To ensure that EC2 instances in private subnets can access the internet for software updates while complying with the security policy that requires instances to be in private subnets, you should use a NAT gateway. A NAT gateway allows instances in private subnets to initiate outbound traffic to the internet but prevents the internet from initiating connections to those instances.

Steps:

Create a NAT Gateway:

Open the Amazon VPC console.

In the navigation pane, choose "NAT Gateways".

Choose "Create NAT Gateway".

Select the public subnet where you want to create the NAT gateway.

Choose an Elastic IP address for the NAT gateway.

Choose "Create a NAT Gateway".

Update the Route Table for Private Subnets:

Open the Amazon VPC console.

In the navigation pane, choose "Route Tables".

Select the route table associated with your private subnets.

Choose the "Routes" tab and then "Edit routes".

Add a route with the destination 0.0.0.0/0 and the target as the NAT gateway ID.

Save the changes.

This setup ensures that instances in private subnets can access the internet via the NAT gateway in the public subnet.