How to update user only if it exists on Mixpanel ?

[rtcoms]

In our case not all users are uploaded on Mixpanel and we don't want to upload all users . Issue is currently we also can't say which are all the users already uploadd on Miapanel.

Now we want to update user info on Mixpanel but don't want to upload any new user on Mixpanel.

How can I achieve that ? Will using append work in this case ? I tried using append , to use for a non existing user and I don't get any error and it returns false for both cases wether user is present on mixpanel or not.

Basically I want to know if a profile is already uploaded on mixpanel or not.