Today, I'll give you an example how to manually edit your boot menu on Ubuntu. The image below is my currently boot list. It's a mess! So, I decided to delete some useless ones.
1- Remove Old Kernel Entries: Just open your Synaptic Package Manage and remove it.
2- Remove Memory test (Serial Console): if you want to remove this entry, you have to delete it. Comment won't change anything. Before changing anything in "/etc/grub.d/" directory, you have to change the permission.
Everything's fine until now. Let's update Grub and watch the result.
Have fun!