If Your phone is Galaxy S4,limited to samsung ,you have to go to system "Settings" select the "Display" option and then select "Font style" option ,choose an available font instead of the default font .Then in the app's "Style Preview" page you can change font by click the one you want to use.Thanks."

26-Font Handwriting&Comic Font Pack for the FlipFont® program will install new fonts on your phone that are compatible with the FlipFont® program on your phone.
This is the author of the latest development of the most comprehensive font straight installed packages directly support Samsung Galaxy series (besides google developer series phones)and all Rooted phone.Perfect replacement font, make your launcher, QQ, micro-channel, meager full of personality
Before using this software, please carefully read the following instructions:
(1) If your phone is Samsung Galaxy series phones, then click on Settings -> Display -> Font Style.
FlipFont program on your phone is compatible.
(2) If your phone is not a Samsung Galaxy series phones, you need to put the phone Rooted, gain system privileges to use this software. For Root, via computer terminal landing http://sj.qq.com/root, which has a convenient one-click Root tool If the phone is already root, just click the font name button, a key substitute fonts.
If you are not satisfied with the font, click the Restore fonts, the software will help you to restore the default font.
Please note If you are using Root way change font, every time you change the font, you need to restart your phone, change the font to succeed.
Disclaimer: This software and FlipFont, independent institutions such as the Samsung Galaxy, all FlipFont, Galaxy trademarks copyrights belong to their relevant institutions.
This android application is not affiliated with FlipFont® or Monotype Imaging Inc. All functionality, trademarks and copyrights remain the property of their respective owners