If you signed up for an account with a non-Chinese Mainland mobile number, email address, or via Facebook Connect and haven’t subsequently linked your account to a mobile number that uses the international dialling code +86 (“Chinese Mainland mobile number”), you are a WeChat user.

In order to switch to become a Weixin user, you will need to link your account to a Chinese Mainland mobile number. You can do so by going to "Me" > "Settings" > "Account Security" > "Phone" > "Change Mobile Number" and binding a Chinese Mainland mobile number.

To learn about the difference between a WeChat user and a Weixin user, refer to My Account > WeChat and Weixin >  “What is the difference between a WeChat user and a Weixin user?”.