RDS PostgreSQL: how to move to the version15?

RDS PostgreSQL can be upgraded as described in the following document.
You said you are using version 9 so once you upgrade to 14.1 you will need to upgrade to 15.2.
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_UpgradeDBInstance.PostgreSQL.html

The upgrade itself can be done manually, and we are aware of no special fees.
Since this will be a fairly large upgrade, we recommend that you take a snapshot and make sure it works well with your application before executing it.

Hi Oleg,

Further to Riku's comments, depending on which version of Postgres you're using you'll likely have to upgrade to 14.1 first before going to 15.2. See this section of the guide for further details - https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_UpgradeDBInstance.PostgreSQL.html#USER_UpgradeDBInstance.PostgreSQL.MajorVersion

Riku is right there are no fees for upgrading, and it has to be manually initiated from either the command line or console.

I hope that helps Oleg :)

Thanks