Is 74LS139 a DeMUX? If yes then how can I give it input and select lines?
In your first paragraph you say that a demux requires:


  2 Select Lines and 1 Input to give 4 output lines


In the logic diagram of the 74LS139, you have exactly that. Two address (select) inputs A0a and A1a, one input Ea, and four output lines O0a, O1a, O2a and O3a.

In fact the image you show from the datasheet explains this in the "Pin Names" section just below the connection diagram.