I am afraid that there is not an easy way to do it without migrating to the latest stack. Indeed, we highly recommend to use this method since it will update many outdated components on the stack (such as Apache, PHP, OpenSSL, Tomcat, etc.)
There's a way to upgrade PHP on the stack without migrating to a new one but it consist on disabling PHP on the stack, install PHP using the system package (apt-get or yum) and adapt the environment so your stack uses the system package of PHP. Please note that will break the out-of-the-box advantage of Bitnami stacks and we do not recommend it.
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