You can only synchronize your online family tree with the Family Tree Builder, where you edit your tree (by republishing it to your family site) but not the other way around.
If you prefer to edit your tree online it will be the other way around, see the following FAQs:
I have complained about this issue numerous times. The best thing would be to have just one place to keep and maintain everything. There are a number of important reasons to do this. Actually I cannot think of even one sensible reason of keeping the present FTB.
You said that if I want synchronisation to occur, I have to work in FTB only. The problem is that there are different tools available on the family site than on the FTB. The last one to appear was to be able to make a smart match for one person on a huge number of family sites. Very nice indeed! None of this newfound data, however, trickles back to FTB. This causes the FTB slowly to rut.
Thank you for responding. I believe that you are improving the software but before data updates go both ways between the FTB and the site there are serious issues. In particular if the site is updated directly without making the same changes to FTB. An update from FTB may nullify or change the once correct data on the site.
If you edit your family tree with the Family Tree Builder, and publish it to your family site, you can't edit the tree online. You can only republish it from the Family Tree Builder to synchronize the tree in your site.
For more information see the following helpful FAQ: