To convert a fraction to a decimal, type it out first, and before pressing Enter, press the diamond key to tell the TI to report the solution as a decimal. This works for all expressions that are expressed as fractions (exact answers).
To change the mode, go to Mode, Page 2, under Exact/Approx., change the value to Exact or Approximate, depending if you want decimals versus fractions all the time. I recommend leaving it on AUTO mode so that you get exact (fractional) answers by default unless you press the diamond key before entering the expression.