I am an M.Sc. student in computer science, working on machine learning.
Recently my supervisor assigned me a paper on biomedical signals field to review. The review task is assigned to my advisor who is a referee of a journal and he assigned it to me to do it for him (I am sure he will not check my review and will submit it directly).
However, the paper is weakly (if not at all) related to my field; I don't even have an idea about its title. What should be my approach? Particularly in judging if the paper has properly covered background works and if it's novelty when I am not familiar with the field at all?
The best option is to decline but I have not that opportunity. That is a "must" to do work for me if I don't want to ruin my relation with my advisor.
I should mention that this is the first time I was assigned to review a paper.
I agree with all of @Nate Eldredge's comments. If you have a good relationship with your advisor, you should be able to say "This paper lies outside of my field of expertise. I don't feel qualified to review it; in fact, even to understand it at a basic level would require me to spend substantial time familiarizing myself with the terminology of a different academic field."
According to your comments, you do not feel comfortable saying this to your advisor. So here is what I would suggest instead: do the reviewing together with your advisor, in person. So for instance you could begin by saying "I started reading the paper you assigned me, and I have some questions I'd like to discuss with you. When can we meet to talk about it?" Then when you meet you can begin to engage with all the unfamiliar stuff and see for yourself how far away it really is from your interests and those of your advisor. If it turns out that your advisor doesn't understand the paper either, I would try to subtly lead him to the conclusion that neither of you should be reviewing it. If he tries to shove it onto you, reiterate that you are doing it but that you want to get his input. If necessary, ask a question of the form "How would you handle this review job if you were in my place?" The point being that the advisor is in your place since he is the one who has been asked to do the reviewing. To pull this off properly may require some verbal aikido.
The worst case scenario is that after all this your advisor makes clear to you that he doesn't care that neither of your have the expertise to review this paper; he wants you to do it anyway. At that point he is asking you to commit academic dishonesty, and you will be in a tough spot: you will have to decide whether turning in a noncommittal review of a paper that you don't understand is the least evil.