OK, first. Ancestry tree will be the most up-to-date. So...
First, unlink FTM tree from Ancestry and delete it.
I usually delete the FTM version from Computer. Then, as you don't want masses of records on your computer twice. (go to documents, FTM, media files and delete them.)
Then, on FTM, start a new tree, and select upload from Ancestry. Then it will show your Ancestry tree - select that and hey presto, you're away.