Unity api level 25 not available

In the Player Settings; the Minimum API level > higher levels are not available are even shown in the list. Mine only goes up as far as Android 6.0 (API Level 23) > when I build apk for my device > it does not load because my device is Android 7.0

My Question >> how can I add higher API levels to my UNITY version…currently I have 5.5.2f1

I have 2 version of UNITY on my MAC

Unity 2017.3.0f3 >>> (this version has Android 7.0 in the “Min API level”)
Unity 5.5.2f1 >>> (this version has Android 6.0 in the “Min API level” only)