They both are interchangeable, so you can use one of them. Just be careful about the ending of the noun that comes after دخل or الى.
دخلتُ الفصْلَ
دخلْتُ الى الفَصْلِ
In the first example we have الفصل ending with a Fat'ha, and in the second example الفصل ends with a Kasra.