The OP here is an example of an XY problem
The title of the thread is asking for help to change GRUB boot selection. But OP appears to be asking for this this to work around a different problem — removing the non-working entry for kernel v5.13. If responses only explained how to change from using first menuentry in GRUB there would still be a non-working boot option in GRUB
There are other other topics on here that explain the different GRUB options
I know of 2 discussions with useful information, but there are many more
How to disable grub2 editing itself on boot, for remembering the last selected entry - #10 by nikgnomic
Help setting default grub entry - #2 by bogdancovaciu
If you can’t find what you need from previous topics you should start a new topic rather than resurrecting an old one.
Kernel releases and EOL changes are published in
Stable Updates - Manjaro Linux Forum