If you use ODROID-C1+ version 4.4.x, You can't watch higher than 360P quality in YouTube.
Open Terminal app and edit build.prop
$ su
# mount -o rw,remount /
# vi /build.prop
ro.build.version.release=4.4.2 -> ro.build.version.release=5
Save and reboot.
Now enable 720p quality.