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.
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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;