How do i get a new head for my character?

First you'll need the head texture item. They can be bought from the shop or other players or other players in the market. Then you can either make a head texture to upload or pay an artist to do it for you. Then you upload it and you're done.

I already have the texture, where shlould i go to upload it?

Go to your inventory (either deactivated or activated, wherever the head texture item is) and click Edit > Browse > Choose file > Upload.

Make sure the head is in the right format too (.png, .jpg, .tga...).