To which extent sum one can modify ones personality?

Answer 1

Character - the distinguishing attributes of a person.

http://www.wordsmyth.net/live/home.php

personality - collective characteristic qualities of a person.

http://www.wordsmyth.net/live/home.php

The Hindu Upanishads say that character is a set of
attitudes of a person at a given point of time.
A person of bad character may turn good at a
later time.

Personality is a collective memory of all the
experiences of past births. It is accessed only
by the sub-conscious mind at the times of crisis.
This is what is capable of converting a person
of bad character into one with good character.

Almighty does not consist of any created material
because all that is created is subject to
annihilation. Almighty is Infinite Consciousness.
The Hindu Advitha theory says that Atma (soul) is a
finite piece of Parama Atma (Infinite Soul). Atma
represents finite consciousness. The Karma sheath
which surrounds the Atma is personality.

One can modify his character during his life time,
but not personality.

Answer 2

I don't know but it's very hard to do. I'm shy and I don't have friends and people always tell me "go out and make friends...talk to people...etc" but it's easier said than done.