My PhD adviser is objecting to my request to change my PhD topic I am a midway PhD candidate, I have finished all of the coursework and the qualifying exam. This my fifth semester (i.e. I'm starting my 3rd year now). Now I feel that, the topic I've selected to work on is not the right one for me. As both my adviser and myself are not experts in that area.

I initially thought things will go fine and progress gradually but it hesn't gone as I expected. So far I have managed to write a manuscript for review paper (almost ready for submission) but I have only got very simple preliminary results. I'm really not feeling comfortable to continue in this topic because I am not getting the guidance that I need. I know that as a PhD candidate, I have a responsibility to do almost research and learn new techniques on my own but definitely help is needed to become an independent researcher, otherwise, we wouldn't have an adviser at all.

I have tried to contact someone in the field from another school but my advisor is not helping me in that regard (which I feel is discouraging). I don't know how am I going to make a meaningful contribution to the field, if I'm struggling to reproduce some results from the literature (that might look basic for the people in the field).

Last week I told my adviser I'm thinking of changing my topic; 


  he told me either I have to continue on the same (current) topic or I need to find a new adviser, he also mentioned that he might leave the school soon so know if I should change to a new topic since this could may take time and delay my progress further.


I'm concerned that changing my adviser at this stage might look negative in the department. I've worked diligently to be allowed to work with my current adviser so I'm trying to avoid that option unless absolutely necessary.

Now, is it expected that I will continue working this topic even if I'm not comfortable with it and my adviser may not help me on it or should I change my adviser regardless of the consequences.

I'm looking for any suggestions on how to convince my adviser to change my topic and work on something that is in our field.
What you need is a realistic option. According to what you write, your advisor may leave soon and your advisor doesn't want to work with you on a different topic. Consider those as given. You aren't going to "wear the advisor down" or impose your own will on the advisor. That seldom happens and even more seldom successfully. 

If your advisor is satisfied with your current progress and expects completion before he leaves then one viable option is to keep on with what you are doing. 

If you really must abandon the topic, then a different advisor is probably a viable option as well. 

I don't see other possibilities here that have much likelihood of success. Either of those two options has implications on time to completion, of course, as well as on how you act and feel in the short term. But either can be leveraged into a career. Keep your eye on that prize. Not the short term.