Please note that Bitnami Stacks are completely self-contained and run independently of the rest of the software or libraries installed on your system. For that reason, it is not possible to upgrade/downgrade one component of the stack (in this case PHP).
You need to upgrade/downgrade de full stack and then restore your files. You should install a new stack with the desired version, and then migrate your data there. This documentation is useful to migrate WP:
These commands will stop your current installation and install the latest WPMS with PHP 7.3
sudo /opt/bitnami/ctlscript.sh stop
sudo mv /opt/bitnami /opt/bitnami.bak
chmod +x bitnami-wordpressmultisite-5.4.2-0-linux-x64-installer.run
sudo ./bitnami-wordpressmultisite-5.4.2-0-linux-x64-installer.run --prefix /opt/bitnami
I hope it helps