During boot you'll see an operating system selection screen. If not, try pressing the left arrow key(←) during the start. Press e within 5s to enter edit mode:
The edit mode looks like following:
Reset Password
You will boot into a special shell, in which the switch_root prompt will appear.
Remount the read-only file system /sysroot in write mode by running the following command:
mount -o remount,rw /sysroot |
chroot /sysroot |
passwd admin |
As the picture shows:
Note that you'll be asked to change again on first login since the password was resetted.