Writing letters backwards is a normal part of developing writing skills in preschool. If you have other reasons to suspect dyslexia (like parents or relatives with dyslexia, or problems identifying sounds or learning to say the alphabet), you should continue to monitor her progress and document your observations in case you see signs of a bigger problem.
Keep practicing with her by doing fun writing activities at home, like writing a shopping list, or writing a letter to a relative. Most of her early mistakes will be part of the process of learning to write, so model the right way, but don't hold her to it too early! She is in an experimentation phase with this skill.