I have an offer from a German university for a masters program starting in the summer semester of 2016.. I like their program and I would like to attend it ( I haven't answered to their offer yet) .
However, I have regretted applying for the summer semester and I would prefer to start in the winter semester of 2016..
Which is going to be their response to this request?? Is it possible to change their offer for the winter semester OR I will have to reapply??

Also, in case I decide to request this, should I explain the reasons that led me to this decision OR I could just mention "personal reasons" ??

Thanks in advance

I'm not sure for Germany but I know in the UK it's usually not a problem to delay the start, unless you leave it until the last minute. I doubt you have to give much of a reason. If you haven't accepted it yet, you could always say you accept it and want to start later.