I have an existing MySQL database running on AWS Lightsail, I want to create a read replica for that one, how can I do it?
To my understanding this is not an option for the Lightsail managed databases. You can however create an RDS instance of MySQL and create a read replica there. To connect to this RDS instance you'll need to use VPC Peering.
Lightsail lets you connect to AWS resources, such as an Amazon RDS database, through virtual private cloud (VPC) peering. A VPC is a virtual network dedicated to your AWS account. Everything you create inside Lightsail is inside a VPC, and you can connect your Lightsail VPC to an Amazon VPC.
To create a MySQL Read Replica:
Using the AWS Management Console, you can easily add read replicas to existing DB Instances. Use the "Create Read Replica" option corresponding to your DB Instance in the AWS Management Console.
Connecting to it can be accomplished via the following method:
In the Lightsail console, choose Account on the top navigation menu.
Choose Account from the drop-down..
Choose the Advanced tab.
Choose Enable VPC peering under the AWS Region where you want to enable it.