how to find find out capacitance using time constant?
The key concept is this line:


  ... we find the point where the voltage reaches 63.2% ...


The time constant τ is how long it takes for a capacitor to charge up to 63.2% of the supply voltage. That is governed by both the capacitance (C) and the resistance (R). τ=RC. That is one of the basic formulae you should know as an Electronic Engineer.

If you know R (which you do because you chose it at 10KΩ) and you know τ (which you do because you measured it) then you can calculate C (τ/R). 

Since τ, C and R are tied together like that it is possible to change the balance of C and R and keep τ the same. For instance, a 10KΩ resistor and a 47nF capacitor would give the same τ as a 1KΩ resistor and a 470nF capacitor. Also the same τ as a 100kΩ resistor and a 4.7nF capacitor.

You can easily change the range without having to change your time measurement system by changing the resistor to give a different scale.