How do I get lightsail's php8 version to change from to php7.4. I'm using lightsail wordpress, but his version is too high and the migration causes incompatibility errors.

Hi.

Lightsail's WordPress image uses Bitnami and PHP downgrade is not supported according to:

https://community.bitnami.com/t/downgrading-php-version/97288

It seems that you need to create a non-Bitnami Lightsail image and set up the required modules (Bitnami, PHP, WordPress) there.

Can that lower version of php be upgraded to a higher version?

Can not. You need to build a new instance in the same way and migrate WordPress.

https://community.bitnami.com/t/how-to-upgrade-my-php-version/96742