Boot your 'running' linux, open a console and then:
Prepare Menu entries
With the information of this output, mount each partition that holds an OS you want to be able to select from GRUB menu. This includes the windows partition which contains C:\, and any further linux you might have installed on this device.
To mount windows, it might look like:
sudo mkdir /mnt/c
sudo mount /dev/sda1 /mnt/c
------------ this part may vary, i'm not on linux atm ------------
Prepare entrie amount
Open up /etc/default/grub, with
sudo nano /etc/default/grub || sudo vi /etc/default/grub
In this file, modify the "default" value, which refers to the default pre-select menu entry at boot.
Save the file and exit to console.
As a last action before you enjoy the new grub, you'll need to rebuild it with:
grub2-mkconfig -o /boot/grub2/grub.cfg
Hope this helps, at least to get started.