Pujya Bapuji said, one should do Navratri upvas for all the 9 days, if one cannot - one should do it for last 6 days, or then also someone can't , one should do upvaas for atleast last 3 days of Navratri, i.e. 7th,8th and 9th day of Navratri.
Special for students
During the Navratri, students should offer an oblation of kheer (rice pudding) to God while reciting Gayatri mantra. This is highly beneficial for students.
For success in work and achieving Raj -Yoga
If you have been recently unsuccessful in job or business or are constantly facing impediments, then take soft tender leaves of Bel and apply red chandan on it and offer it to Goddess Mother Jagadamba on the eight lunar day of Shukla paksh. Then recite this mantra, "AUM HRIM NAMAH", "AUM SHRIM NAMAH". Then sit down quietly and do your prayers and japa of Guru Mantra which will translate to Raj-Yoga. It is even more beneficial to do this during Navaratri. This has been advised in Devi Bhagwad by Sage Vyaasa.