Click on the VPS you would like to reinstall:

Then click on Reinstall:

Choose the desired OS then click on Reinstall:

Please make sure to take note the password because it would be shown 1 time only!
After that wait 5-10 mins for the installation to finish. You can also check the process by using the VNC feature.
With CentOS 7, please run "resize2fs /dev/vda1" to extend the hard disk