Continuing from veganist answer,
1. Log in to Cpanel
2. Sign in to PhpMyAdmin
3. Click the database that is used
4. On the page there is a list of tables in the database, select a table wp_users by clicking on the left side of the table.
What appears next is a table with the field as a column name and data content, namely WordPress blog user data. If there are several, make sure the user where you want to change your username to login and / or password. Then click on the pencil icon to edit
5. By clicking the Edit icon, it will open the page to change the selected user data.
And to change the password, select the row user_pass, then:
- Function> replace it with the MD5 method
- Value> enter a new password. For example: Johnny15483Science
After that click the Go button to save the password change.
Furthermore please try to log into your WordPress blog with the username or password new. Good luck