Hi @littledevil
The onclick parameter is used for Javascript, not PHP. So your code won’t do anything because it won’t call your PHP update_ranks() function. Please learn more about it here: http://www.w3schools.com/jsref/event_onclick.asp.
If you want to update the field from the front end, I suggest you use the acf_form() instead. This page should give you more idea about it: https://www.advancedcustomfields.com/resources/create-a-front-end-form/.
If you’re not familiar with programming, I suggest you hire a developer to help you out with it, and I’d recommend looking for one on https://studio.envato.com/ or https://www.upwork.com/.
Hope this helps.
