Change Font Family

Hello, First of all: sorry, for my very bad english ;) I have a problem with changing the font family. I want to change the "section title font family" from the default font "great vibes" for example in "helvetica" but the font turns into the "comic sans". no matter what I choose, the font turns always into the comic sans. I hope you unterstand my problem and have a really easy solution. Please help me, I dunno how to fix this. best regards