Sometimes astrological predictions(Jathakam) is entirely bad thought out the life. Some astrologers tell this person's life is always doomed.
Can he/she change it in the present life itself by doing good karmas?
The Astavakra Samhita 1.11 says (Swami Nityaswarupananda translator):
He who considers himself free is free indeed, and he who considers himself bound remains bound. 'As one thinks, so one becomes' is a popular saying in this world, and it is quite true.
If you think you cannot change your karma, you cannot. If you think you can, you can.