You can change your name only once a day.
On a mobile device:
Click on the profile icon at the bottom of the screen and then on the “Edit” link under your name — you will be taken to the “Editing” menu where you can change your name and other personal details.
On a computer:
Click on your profile picture at the top of the screen > a drop-down menu will appear where you can find the settings > select “Personal information” and change your name in the first field.