about job timing

Sir I work with Tata 1 Mg from last year when I was recruiting so I ask to my hr what is working hours so he said that 6:00Am to 3:00PM. Then I join the company. 3-4 Month ago company introduced evening which timing is 10:00 Am to 7 PM that time I talk to my city manager he said in your offer latter there is no timing mentioned so you have to do work otherwise you can leave. Now today company change another shift which is 6:00 am to 10:00 am and 3:00PM to 7:00PM I don't know what to do I am mentally disturbed because I am field worker and work as a phlebotomist (home blood collection) please tell me what legal thing I can do or anything