Username should be chosen with care for while it is possible to change it - it may have implications beyond recognition.
If you still think you are up for the change - please read on.
By reading the man pages you find the GNU system has thought of it.
man usermod man groupmod
You cannot login as the user you want to change - you must use root.
Manjaro usually creates a user group matching the username and assigns the user to the specific group. This makes it necessary on Manjaro to rename both the login and group.
When you are logged in as root first rename the user and rename the user's home
# usermod --login $NEWLOGIN --move-home --home /home/$NEWLOGIN $OLDLOGIN
Then rename the user's group
# groupmod --new-name $NEWGROUP $OLDGROUP
NOTE: This can also be done from a live ISO using chroot.
Contributed by @freggel.doe at Manjaro Forum
Some symlinks in that new home-directory might be broken after, because their target contain the full path with the old username. Wine and it's default
$HOME/.wine -prefix comes to mind.
To find dead symlinks:
find /home/$NEWLOGIN -xtype l
Some config-files contain full paths as well (desktop background image for example). To find files with "old" username:
grep -rn "/home/$OLDLOGIN/"