[AT&T Samsung Galaxy S6 Edge+] meek2100:
You can update your phone without an ATT SIM card you will just need to ODIN your phone to 2AOJ1, then use ADB sideload to go from (5.1.1) 2AOJ1 -> (5.1.1) U2APB2, and then use ADB sideload again to update to MM 6.0.1. (Their are 2 builds available U2BPD5 or 2BPE1) You can choose which you want to try, U2BPD5 is proabably a little more stable since it is the version that was listed on Samsungs support site a week ago.

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------

EDIT: Instructions on how to update ATT Samsung S6 Edge+ (G928A) to Android 6.0.1 (Marshmallow)

1. Back up your phone with Smart Switch. When you sideload the update it will wipe your phone.

2. YOU MUST HAVE build G928AUCU2APB2 on your phone. Check Settings > System > About Devices > Baseband version.

2A. ODIN back to 2AOJ1

(5.1.1) 2AOJ1 ODIN files for clean flash or restoring to STOCK. Credit to: Yamaha169

https://drive.google.com/file/d/0B1dPgClURCHJTjA3dF82WnNiMzg/view​

2B. Follow steps 3-11 to update to adb sideload U2APB2 and then follow 3-13 to update to 6.0.1

(5.1.1) 2AOJ1 -> (5.1.1) U2APB2 Credit to: bcak610

https://xdmd.sl.attcompute.com/agents/48726/1488/SS-G928AUCU2AOJ1-to-U2APB2-UP​

3. Download & install 15 second ADB.

http://xdaforums.com/showthread.php?p=48915118#post48915118

4. Download update files from ATT's server.

(5.1.1) U2APB2 -> (6.0.1) U2BPD5 Credit to: GLIMMERMAN76 and bcak610

https://xdmd.sl.attcompute.com/agents/48726/1488/SS-G928AUCU2APB2-to-U2BPD5-UP

5. Rename the downloaded update file to "OTA.zip" and move it to your main PC directory C:\

4. Open "Command Prompt" on your PC

6. Shutdown phone.

7. Restart phone by holding down the POWER, HOME, and UP button at the same time. DONT release until you get an UPDATE LOGO on your phone.

8. Use the volume keys to highlight "Apply update from ADB", then press the power button to select it.

8. In the command prompt window, type adb devices.

9. Type adb sideload C:/ota.zip Change C:/ to match the directory you moved the update file to.

10. Press Enter and it should start transfering & installing the OTA file

11. This process will take 1hr so be patient and let the phone reboot.

12. Once the phone reboot, you will have 6.0.1!!!!!

13. Restore from your Smart Switch back up.

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------

EDIT: G928A Update File Links

(5.1.1) U2APB2 -> (6.0.1) 2BPE1 Credit to: meek2100 NOTE: Android pay doesn't work in this build.

https://xdmd.sl.attcompute.com/agents/48726/1488/SS-G928AUCU2APB2-to-U2BPE1-UP

----------------------------------------

(5.1.1) U2APB2 -> (6.0.1) U2BPD5 Credit to: GLIMMERMAN76 and bcak610

https://xdmd.sl.attcompute.com/agents/48726/1488/SS-G928AUCU2APB2-to-U2BPD5-UP

----------------------------------------

(5.1.1) 2AOJ1 -> (5.1.1) U2APB2 Credit to: bcak610

https://xdmd.sl.attcompute.com/agents/48726/1488/SS-G928AUCU2AOJ1-to-U2APB2-UP

----------------------------------------

(5.1.1) 2AOJ1 ODIN files for clean flash or restoring to STOCK. Credit to: Yamaha169

https://drive.google.com/file/d/0B1dPgClURCHJTjA3dF82WnNiMzg/view