在网上搜了好几篇文章，要么不全面，要么升级地址不正确。所以自己摸索着升级成功了，记录下来，给其他需要的朋友一个帮助。
step 1 :
yum -y install wget* mkdir -p /root/upgrade cd /root/upgrade wget http://dev.centos.org/centos/6/upg/x86_64/Packages/preupgrade-assistant-1.0.2-33.0.3.el6.centos.x86_64.rpm http://dev.centos.org/centos/6/upg/x86_64/Packages/preupgrade-assistant-contents-0.5.13-1.0.5.el6.centos.noarch.rpm http://dev.centos.org/centos/6/upg/x86_64/Packages/preupgrade-assistant-ui-1.0.2-33.0.3.el6.centos.x86_64.rpm http://dev.centos.org/centos/6/upg/x86_64/Packages/redhat-upgrade-tool-0.7.22-3.el6.centos.noarch.rpm
step 2 :
yum -y localinstall preupgrade-assistant-*
step 3 :
preupg
step 4 :
yum localinstall redhat-upgrade-tool-0.7.22-3.el6.centos.noarch.rpm
step 5 :
rpm --import http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-7
step 6 :
redhat-upgrade-tool --network 7.0 --instrepo http://mirror.centos.org/centos/7/os/x86_64/ --force
step 7 :
reboot
step 8:
cd /mnt/var/lib/rpm rm __* init 6
step 9 :
cat /etc/centos-release
CentOS Linux release 7.0.1503 (Core)
success!