Backup Lightsail Instance Step By Step, Follow the These Simple Steps with screenshots to better understand.
Open the main Lightsail Instance Home Page Click on your Instance, You will see a Tab With 8 Options

Give Snapshot a name

Do Not Click Anywhere or Refresh the Window

To create a new Instance from a saved snapshot.

Create Automatic Backups in AWS Lightsail

Taking Backup of your Lightsail instance is simple and can be automated, you can get started with Lightsail for Free!
Bonus Tip
Always keep a snapshot for your configuration of a lower level to degrade the instance, because you may always upgrade to a higher configuration instance, but you can’t degrade.
With Configuration, I mean CPU’s & RAM, which means you may upgrade to 1Gb Ram from 512MB instance, but you can not degrade back to a 512MB instance with a snapshot of 1GB.
In one of my posts on WordPress, I have explained why you should never install cheap plugins without developer support on your WordPress sites.