Since you can only log in as pi, you can't modify user pi when logged in as pi, but you can use your sudo priviliges to create a new user
then it'll ask you for a password twice
Then you'll need to add it to the sudoers group with
Then copy the entry for pi changing it to your new id
then writeout (CTRL + O)
CTRL + X to exit
Then make sure logging in with the new ID works and you have sudo powers.
Then you can remove the pi user if you want.
but if you want to remove the /home/pi directory you can use
You'll have to go back into the sudoers file to remove pi as well.