If you received a “Sync Conflict” error message (see picture below) when trying to sync your family tree on Family Tree Builder (FTB) with the site, it means that there is a sync revision mismatch and a one-way sync is needed.
Aside from all the technical reasons, the message gives you two options to solve the problem: “Download” and “Upload”.
Download means that you will bring the family tree from the site and it will overwrite the current family tree that you have in the FTB.
Upload means that the family tree you currently have in the FTB will be sent to the family site online and overwrite the current tree you have in there.
You should read the error message carefully and understand which version - the online version or the FTB one - has the highest number of people and choose that one to be synced and avoid data loss.