1) Download HxD for your language from http://mh-nexus.de/en/downloads.php?product=HxD. I use the portable version so I do not have to install it.
2) Extract the program from the zip file.
3) Go to your FIFA 15 install folder and make a backup copy of fifa15.exe or fifa16_demo.exe
4) Run HxD.exe and open up fifa15.exe or fifa16_demo.exe
5) Click on Search then Replace in the menu
6) Enter "39 8E E3 3F" in "Search for" and "8E E3 18 40" for 3440x1440 or "26 B4 17 40" for 2560x1080 in "Replace with"
7) Change "Datatype" to "Hex-values" and click "Replace all"
8) Save your changes and you should be good to go.
9) If you have issues replace the fifa15.exe or fifa16_demmo file with the backup you made in step 3