As mentioned by @Scott in the comments under your question:
Your here is the right link. Use the bottom answer to rename the
bin/php directory. The MAMP preference pane will show the 2 latest
versions in the php directory. – Scott Eisenberg
And the answer in that links is:
First stop the Server if its running. Go to "/Applications/MAMP/bin/",
rename the PHP Version you don't need (MAMP is only allowed to use 2
PHP Versions), e.g. "_php5.2.17". Now MAMP will use the php versions
that are left. Go to the MAMP Manager and then settings, then switch
to the php version you need.
In my case (and similarly yours), I renamed the folder named php7.1.1
to _php7.1.1
and now MAMP shows me 5.6.30
and 7.0.15
in the preferences pane.