When Predictive text option is ON, text input may sometimes be awkward as it suggests/replaces words while typing. To correct this, set “Predictive text” option to OFF. You can also set the Recapture function on Predictive text menu to ON, and the revised word will be saved, too. This can be used to overwrite words that have been previously saved incorrectly.
To do this, go to Messages and then press the Settings button. Look for Predictive text and make sure to turn it off.