Navigating to the earliest/latest command in history when you’re in the middle of your history

Take a look in the man page:

man bash

Here I copied for you the thing you were looking for:

previous-history (C-p)
    Fetch the previous command from the history list, moving back in the list.  
next-history (C-n)
    Fetch the next command from the history list, moving forward in the list.  
beginning-of-history (M-<)
    Move to the first line in the history.  
end-of-history (M->)
    Move to the end of the input history, i.e., the line currently being entered.