Swap Postdoc position I was offered a post doc position in a top level university about 2 months ago, which I have already accepted. 

However, I was recently called for an interview in another equally prestigious university for another postdoc position which I applied to before getting the previous position offered. 

The second position is, in my view, much better than the first one in many senses (I like the country more, a substantially higher salary, closer to my home town), however I am worried that if I decline the first offer (which I already accepted) now, the supervisor from the first university will get annoyed and that will carry repercussions in my future career and my image to the scientific community. 

I have heard that many people do these things (declining offers already accepted) for academic positions, but that this is seen as much worse for postdocs positions. However, I think that everyone would agree that the second offer is a substantially better option, even the supervisor from the first university. 

Should I give up the position I already accepted in order to pursue the better option?
To come right to the point: after accepting the first offer, you should have withdrawn all your other applications.  When you were contacted for the second interview, it was not enough to inform them that you had already accepted an offer -- you should have informed them of this and therefore declined the interview.  (I find it strange though that this second institution was happy to interview someone who told them they had already accepted another offer.  I wonder if there was some kind of miscommunication / misunderstanding here.)  In academia, accepting an offer means committing to show up to that job for at least one semester (which is the atomic unit of most academic jobs).  In my circles at least, the minimum length of time to spend in an accepted academic position and "leave honorably" is one academic year.


  I have heard that many people do these things (declining offers already accepted) for academic positions, but that this is seen much worse for postdocs positions.


Yes, it happens somewhat frequently.  Like a lot of negative human behavior, that it happens frequently does not excuse it.  I don't necessarily agree that it is seen as worse for postdocs; it really depends on the situation.  I would argue that all other things being equal, getting reneged on by a temporary employee is not as bad as getting reneged on by a (relatively) permanent employee.


  However, I think that everyone would agree that the second offer is substantially a better option, even the supervisor from the first university. 


Better for you.  But would the supervisor from the first university think it is better for her?  What happens to her if you renege -- will she have the time and opportunity to hire someone else?  If she can't hire anyone, will that have negative repercussions on her work and the work of her affiliates and students?  

The way to find out the answers to these questions is to ask.  I strongly suggest contacting your supervisor as soon as possible.  Explain your situation and perspective, including why the second position would be substantially more desirable to you for personal reasons.  Then ask what repercussions there would be if you weren't able to show up for the first position.  If there would be severely negative consequences for your supervisor, I think you should stick with the job you committed to.  Otherwise you may suggest to the supervisor, with maximum politeness, that you would like to take the second job instead of the first, and see if she will give you her blessing.

Good luck.