If your account is linked to a China's mainland mobile number (+86) or a QQ number, you are a Weixin user. To switch to a WeChat account, you must link your Weixin account to a non-China's mainland mobile number.
To do this, follow these steps:
Go to 「Me」>「Settings」>「Account Security」>「Phone」> Tap「Change Mobile Number」and bind a non-China's mainland mobile number.
For more information about the difference between WeChat and Weixin users, refer to My Account > WeChat and Weixin > "What is the difference between a WeChat user and a Weixin user?"