Changing WordPress username in phpMyAdmin requires a little more knowledge but a lot less messing around than the other two mentioned methods but if you want to try them you can find them on wp beginner
1. In phpMyAdmin find the database to change and click on it, then click on phpMyAdmin:
2. Click on wp_users:
3. Click on Browse (and possibly Structure then Browse again) if you can’t see ‘Edit’ below the fields:
4. Click on Edit:
5. in the row [user_login] type your new name using numbers and capitals too in the box in the column ‘value’, then click on [Go]
This is especially useful and necessary for brute force attack hackers trying to guess your password if they already have your easy to discover username. Easy how? Well with the wordpress 4.7 (and some of the older versions too I think) came this little bit of fun: adding /wp-json/wp/v2/users to your websites url will display all the usernames!!!!
After the name change in phpMyAdmin, the username displayed is the ‘nice name’ (screen display name) not the username. Phew.