The Variable which is already bounded then you can't bound it again. In Erlang operator (=) is used for pattern match not for initialization. But If the Variable is unbound and we use operator (=) then data on the right-hand side will be bound on the left-hand side Variable.

If you want to bound another data then you need to unbound that Variable first.

In Erlang Shell you can use f() function to unbound all the Variables. and to unbound any particular Variable use f(Variable_Name).

For Example:

> One = 2.
2
> f(One).
ok
> One = 1.
1

Now, Variable One is bounded with value 1.

That's it.