I think the newer kernel should work after reinstallation.
If it doesn’t and you want to stay with the older one:
apt list --installed linux-image*
There should be a package with a specific version number in its name. For example, the standard kernel for Debian 11 is: linux-image-5.10.0-26-amd64
Uninstall the linux-image-… package you don’t want to keep.
Also uninstall linux-image-amd64 which is the meta-package that pulls in the newest kernel version. Without it, you won’t get new kernel versions in upgrades.