For an existing game in progress... you'd go into the game preferences and change the options for "Text Speed" and "Auto-Forward Time".
Text Speed at 100% means the text appears instantly. Anything less than that will make the text appear one letter at a time.
I tend to have mine at around 10% to 15%, which is fractionally quicker than my reading speed.
Auto-Forward Time set sets how long a bit of text dialogue will remain on screen before automatically advancing to the next line of dialogue... if you have AUTO switched on. (I do). There's a big long calculation involved, including how long each bit of dialogue is. But this settings affects how fast or slow the game auto advances.
If you're talking about a game of your own, where you want the defaults for new players to be how you prefer things, then you can override things with values like these two lines:
Python:
# --- options.rpy ---
default preferences.text_cps = 36
default preferences.afm_time = 8
This will only affect new play-throughs the first time played. After that, the game will use whatever values were for the latest play session. i.e. leaving the player's own choices in place.