To contact help center of facebook app in your smartphone, you need to follow these steps:
First of all go to the menu bar of facebook in smartphone. Scroll down and you will find an option named as "Help & Support"
Click on it and it will show you many options. As we need Help Center then at the first option there is written "Help Center". Click on it.
At next stage you will find the page showing all the faqs and you can find as required by you.