First, you must have a texture for your head. You can make one or request one
here
Second, once you get your texture for your head, time to upload it to your tori. You go first
here. You will see that there is a word " Head Texture ".
Third, Browse to your folders and get the file or texture for your head. You click it and press " Upload "
Fourth, once this is done, wait 5-10 minutes or more depending on your internet connection. After this, you will see your texture on your head.
Congrats.
If you want a new texture, just repeat the process.