To boot to your desired operating system you will get the selection menu at the start up which will allow you to select the operating system on which you wanted to boot the system. If you are not getting the boot menu then let me know does it throws any error while booting. I have one solution which might work with the issue. Just use the Live CD and use the below given command and see whether the issue got resolved or not.
Code:
ubuntu@ubuntu:~$ sudo update-grub
Also to know about the more regarding the installation of then Grub visit the below given link.
Grub
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