In your scene go down your avatars Armature until you get to the head and just drag it there from your project assets :) You will have to adjust it there but it should move with your head and whatnot.
Just drag it into your armature to where you want it to be (Head for example). You might have to adjust it to get it on right but that should be all you need to do.
Hey Peanut! All you need to do is go into the Materials folder after you have added the package, Choose the material (there's one for the body, backpack and the heart), after you choose one on the inspector tab, (its usually on the right side panel) there will be a small picture next to the title "Main Texture". Clicking the little circle between that text and the image would bring up a select texture screen. Just choose the colour you would like and bam done! There are quite a few other ways to do this as well.
Ah i included a textures folder with all the alternate colours and faces, if you go into the materials folder there are materials for the emote faces, body, backpack and heart. You can just change them up in there :) You just click on the material and then choose a new one from there.