Hit the account selector menu at the top of the screen, and then click the three vertical dots next to the account you want to edit. From here, select 'Account details', click the pen icon next to the account name, and edit the text field. Once you're done editing, click the checkmark to save.
Tap the ellipsis to the right of your account address, and then select 'Edit account name'. From here, just enter the name you'd like: