GUIDE BY GNΩSIS : Moddb.com
VISIT THIS STEAM SITE FOR FULL GUIDE BY GNΩSIS : Steamcommunity.com
Here's Why The Assassin's Creed I Overhaul Mod is a texture overhaul mod by Hecumarine, which updates a lot of the old textures from Assassin's Creed to look more like Assassin's Creed: Revelations. Some people find the old textures off-putting, and while I personally don't think it's that bad, nevertheless, this mod really does a great job at making Assassin's Creed look better. 
When I first got the mod working, I had two main convience issues with the recommended installation for Steam:
1. When I renamed my OpenTexMod.exe to AssassinsCreed_Game.exe (replacing the original) and launched Assassin's Creed from Steam, this launched OTM as intended. Though upon launching the actual game with OTM, Steam would not register me as in-game nor would Steam Overlay be enabled (If I wanted to view guides, take screenshots, etc.)
2. If I instead used the original AssassinsCreed_Game.exe for launching and simply opened OpenTexMod.exe before-hand, my game would always automatically launch using AssassinsCreed_Dx10.exe rather than AssassinsCreed_Dx9.exe, and Dx10 doesn't work with the Overhaul Mod.
In this guide, I'll provide a installation method which works around these two issues very conviently.
Installation Instructions
1. Download the mod package from ModDB,
2. Extract the mod package to a location of your choice.
3. Copy or move the main .tpf file (Assassin's Creed Overhaul 2016 Full Version.tpf) and if you want, any additional .tpf files from the "Optional Plugins" folder into the Steam directory for Assassin's Creed.Usually
C:\Program Files (x86)\Steam\steamapps\common\Assassins Creedor C:\Program Files\Steam\steamapps\common\Assassins Creed
4. Copy or move the contents of the OpenTexMod folder included in the package into the Steam directory for Assassin's Creed. 5. From your Steam directory for Assassin's Creed, rename AssassinsCreed_Dx10.exe to something else like AssassinsCreed_Dx10_Original.exe.And rename AssassinsCreed_Dx9.exe to AssassinsCreed_Dx10.exe .This way Steam will always run the Dx9 version of the game, you can skip all of this if your Steam runs the Dx9 version by default.
6. Now, from your Steam directory for Assassin's Creed, Run OpenTexMod.exe.
7. Click "Main" > "Add game" and add AssassinsCreed_Dx10.exe (the one that's secretly Dx9). You can now exit OpenTexMod.
8. Open your Steam client. On the bottem left corner click "Add a Game..." > "Add a Non-Steam Game..." > "Browse" then find and choose OpenTexMod.exe.
9. Now, each time you would like to enable the Overhaul Mod with Assassin's Creed, simply launch
OpenTexMod from Steam.
10. Then launch Assassin's Creed. Alt + Tab to switch the window back to OpenTexMod which has magically detected that Assassin's Creed is running. Now click "Open Texture/Package" and add in the .tpf files you would like to enable that you've added to the Steam directory for Assassin's Creed. Wait for each file to finish loading.
11. Click "Update (reload)" and wait for the reload to complete.
12. Done! You can now return to the game and enjoy the modded game with Steam overlay!
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The steam overlay is needed ?
Would be nice if someone could do one of these for non steam versions as I have the same problem. Instructions are either unclear or incorrect, because they seem quite simple to follow yet don't work to get this mod working.
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What if you have it on Uplay? How would I do this then?
Confirmed, the easiest is to use the Old TexMod BUT first upgrading the AssassinCreedDX9.exe to work with more than 2GB of RAM using CFF Explorer, no need to play around with anything else than these 2 simple steps:
0. MOVE FILES IN AN EASY-TO-ACCESS GAME SUBFOLDER: the large Textures .tpf File, the TexMod.exe and the CFF Explorer program Folder
1. ACTIVATE MORE THAN 2GB RAM(Step necessary only once): From CFF Explorer open the AssassinCreedDX9.exe, then Click on "File Header" subfolder, then click on line "Characteristics --> Click Here", then check the checkmark "App can handle more than 2GB address space", then DON'T FORGET TO CLICK SAVE AND OVERWRITE! ;)
2. RUN GAME WITH NEW TEXTURES: From TextMod open the AssassinCreedDX9.exe, then from window "Select Packages" select the large .tpf Textures File, then click "Run".
3. OPTIONAL: SETUP START FROM STEAM INSTEAD OF FROM FILE EXPLORER: Steam "Play" button uses AssassinCreed_Game.exe which uses AssassinCreed_Dx10.exe... so trick it and delete the Dx10.exe and rename the Dx9.exe into Dx10.exe... ;)