Changing the fonts on your galaxy s6 is really not different from changing the font on other android phones. Just go into the settings click on display and click fonts. There will be 5 different fonts to choose from. However a cool trick is to click the download fonts option at the very bottom. From in here you can choose from a wide variety of different fonts that are mainly paid fonts that cost $.99 to purchase.
There is another way to get some cool free font styles for your samsung galaxy s6. Simply go to the playstore and type in fonts. There will be a list of a bunch of different font apps. I recommend checking out the top one hi fonts which is very cool and best of all free for download.