You want option one,
As long as you haven't done anything with the windows partition during the manual partition step it should be fine and the centos install wont touch it. but it will overwrite the old centos 6 install
but if you just want to upgrade from 6.4 - 7 this may be a better option for you
http://linuxsysconfig.com/2014/07/upgrade-to-centos-7/
it's the guide I used to do mine