I had windows 10 in my lenovo V110 laptop, Legacy BIOS. Then I installed Loki alongside windows in another XFS drive. Now after installation, whenever I start my computer, windows 10 opens directly instead of OS choice prompt.
Well, the simpliest way is using Boot-Repair utility.
You can find the instructions on official website. There are several options such as preparing bootable stick with special image, or installing it on live session and using from there.
P.S.: be careful with advanced options.