Have 16.04, 14.04, and Want to Remove 15.10

[scpatl4now]

I had some free space and wanted a clean install of Ubuntu 16.04, I want to maintain 14.04 for a while longer and play with 16.04. I also have a 15.10 install I want to get rid of and combine that space into the 14.04 partition. I know how to partition disks, but when I delete the partition with 15.10 I know I need to update grub and was slightly unsure how

[kansasnoob]

Which OS boots by default if you make no manual choice from the grub menu?

[scpatl4now]

16.04 is the last one installed so it is first in the grub menu