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 one component of the stack (in this case PHP). You need to upgrade de full stack and then restore your files. Check this case
Note that PHP 7.0 is no longer maintained. Due to this, WordPress Stack ships PHP 7.2, as their developer recommend.
Please let us know if you find any issue following this process.
I hope it helps