I have Windows 8.1 Lenovo S510P laptop on which I installed Ubuntu 14.04 LTS.
But not able do boot repair, [or] edit grub; so when I power on laptop I should get option to go Windows or Ubuntu.
Right now I can only able to login to Ubuntu.
You probably need to update grub, which can be done by running sudo update-grub. This usually is caused by grub failing to recognize Windows 8.1. Worse case senario you might have to reinstall grub entirely. To do that, see this link;
http://howtoubuntu.org/how-to-repair-restore-reinstall-grub-2-with-a-ubuntu-live-cd
sudo update-grubIs there still a Windows 8.1 partition found? – david6 Nov 25 '14 at 9:57