Pressing the keys Ctrl and \ at the time time while in the VRChat application will set you to a grey robot avatar locally, which will allow you to change to a different avatar.
In cases where the option above isn't available, you can navigate to the VRChat website, go to the Avatars section, and click the button "Reset to Default Avatar". Here's a link: https://vrchat.com/home/avatars
If you are using a Steam or Oculus account, you will not be able to sign into the VRChat website. In those cases, create a new Support Ticket so that VRChat Support can help reset your avatar for you.