I finished a story-only play through of Assassin's Creed: Rogue, and want to go back and finish everything. How can i do this? Do i need to start over again to get 100% Sync on my memories?
Just wait for the credits to roll over (not possible to skip, unfortunately, and they are quite long) and you should be able to continue getting to your 100%.
After you're in freeplay mode you can select memories to replay in escape menu, as far as I remember.