The AWS Elastic Beanstalk Console currently only permits you to change between minor platform versions (e.g. from 64bit Amazon Linux 2 v3.1.6 running PHP 7.4 to 64bit Amazon Linux 2 v3.2.1 running PHP 7.4), but does not allow changes between major versions (e.g. from 64bit Amazon Linux 2 v3.2.1 running PHP 7.4 to 64bit Amazon Linux 2 v3.2.1 running PHP 8.0).
But, fret not!!! It is possible to update to a major platform version using the AWS Command Line Interface (CLI):
aws elasticbeanstalk update-environment --solution-stack-name "64bit Amazon Linux 2 v3.2.1 running PHP 8.0" --environment-id "x-xxxxxxxxxx" --region "us-west-1"
To find the newest PHP platform version AWS Elastic Beanstalk supports view the Elastic Beanstalk Supported Platforms.
For previous versions see the PHP Platform History.