@terziyski thank you for answering. I had followed it and it didn't detect the need to update it. And when you force with the method 2 you cannot select which deco you will update. It would update all Decos in that network no matter version they are. So it would install firmware v2.6 into v1.6 and it could damage them. 

WORKAROUND : So i did the hard way by creating a separate network with the Decos XE75 v2.6 only and updated them using the method 2. See the steps:

1. I removed the new v2.6 decos XE75 from my network.

2. Connected a network cable at the Main Deco v1.6 and the other end ar v1.6. 

3. Created a second network by clicking on " + " " Create Network ". 

4. Added the second node v2.6. 

5. Verified the FW and it again couldn't detect the need to upgrade the FW. 
6. So then i followed the Method 2 of the link https://www.tp-link.com/support/faq/1599/

7. I downloaded the firmware 2024 into my phone. 
8. Unzipped the file.  

9. Selected the bin file and clicked to update. 

10. It updates all Deco in the second network. It didn't update the original network of v1.6. 

11. All v2.6 updated, i removed them from the second network and added them back into the original network. 

Now how is gonna be the future ?? Whenever a new firmware should i repeat the workaround ? 

I hope the TPLink engineers could respond it.