If I am not wrong then some of your might searching about tips so that you can pass the preli exam. 

Actually you already know what you have to do but you are not doing it instead your just searching on online and wasting your time.
You don't have enough time now. So from now what should you try,/ what should you do for the next upcoming exam of BUET preli?

Well if you haven't started then you should start practicing MCQ from the test paper or the previous question bank of admission. After trying them then again read all the theories, properties etc. For Chemistry you can just read Hazari sir's book. Focus on that subject that that is important and was given in the previous exams of preli.

Pray, read, work hard and ask from Allah. 

Now don't waste your time, go and just start don't worry about what will happen next! Believe and stop worrying. Just think, it just an exam.