Then I'm sure it's the keyboard. If the predictions appear in exactly the same way (identical boxes above the keys) when using any other app where you enter text that would imply it's the same app providing them.
In which case it depends what keyboard you use (if you've not installed one, on what keyboard came with your phone). But the answer will be to look in the keyboard's settings for an option to edit the custom dictionary. Most have it somewhere.
To find the settings, either look for a key with a gear symbol or similar on the keyboard and press and hold that, or go into system settings > Language and Keyboard and look for the keyboard settings there.