To delete your save file from the PC version of Assassin's Creed: Unity, please delete the 720 folder (Uplay) or 857 folder (Steam) found at the following location in your PC's file directory:
Steam - C:\Program Files (x86)\Ubisoft\Ubisoft Game Launcher\savegames\[Uplay ID Number]\857
Deleting the 720 or 857 file and launching the game again should allow you to start a new save game.
Please note that you may need to disable the cloud save functionality within Uplay to start a new save file without the old save file being re-downloaded and overwriting it. Find out more information in this article.
Now you can delete your save data and start a new save file. Once the new save file has been created and saved locally, you can re-enable the cloud save. The next time you launch the game with the cloud re-enabled, you will be prompted to choose between the local and cloud save file. Choose the local save file to upload that to the cloud so there will be no more save file conflicts.
To delete your save file from the Xbox One version of Assassin's Creed: Unity, please follow the steps outlined in the guide found on Microsoft's website.
To delete your save file from the PS4 version of Assassin's Creed: Unity, please follow the steps outlined in the Sony's user guide for system storage and for online storage.
If you need any assistance regarding this process, do not hesitate to contact us.