When you first log into AltspaceVR, you will be using a Guest account with the default avatar.
In order to change this, you'll need to either log in to an existing account or sign up.
Once you are in a logged-in state, you are able to change your avatar. Select your desired avatar and color scheme by clicking the blue 'Customize Your Avatar' on the left side of your main menu window.
You will notice there are an array of avatars you can select from. Both robot and human looking avatars are available. Some avatars have different degrees of customization.
Note: that if you are using Altspace in 2D mode on an Android device, you will not be able to pick you avatar. Please proceed to use VR mode with a supported GearVR or Daydream headset, or download the 2D mode client on your PC.