In this week’s Torah portion, R’eih, we read: 
Be careful to observe only that which I enjoin upon you: 
neither add to it nor take away from it. (Deut 13:1)
If we examine Jewish history, we observe that the Jewish community has both added to and taken away from the laws as found in the Torah – whether  it’s the sages of the Mishnah and Talmud, the rabbis of the Shulhan Arukh, or  the modern leaders of the Orthodox, Conservative and Reform movements.

So how can this be?

Throughout the ages, we have attempted to apply the wisdom and overarching principles of the Torah to the realities and details of life. We have done this through a rich and long history of biblical interpretation and commentary, and legal decisions. 

Perhaps Pirke Avot (5:22) explains it best: 
Ben Bag Bag used to say: Turn it over and over because everything is in it. 
Reflect upon it and grow old and worn in it. 
Do not leave it, for you have no better lot than that.