Here's how you can downgrade iOS 11.2.6 to iOS 11.2.5 on iPhone, iPad or iPod touch. Also, here's why you shouldn't do it.
Complete Tutorial on How to Downgrade iOS 11.2.6 to iOS 11.2.5 on iPhone or iPad
Before we get into the downgrade tutorial itself, it's important to mention what iOS 11.2.6 brings to the table. On the features front, there is absolutely nothing new at all for users. Instead, this is a bug fix release. It patches something so dire (and stupid) that it will leave your device absolutely useless. It's a simple case of sending a specific Telugu character via iMessage or anywhere else and the entire iPhone or iPad will be rendered useless for a moment. So that's why it's an extremely bad idea to downgrade iOS 11.2.6 to iOS 11.2.5. If you understand this very well and still wish to go ahead with it, then follow the guide outlined below.
Before you go ahead, make sure you download iOS 11.2.5 firmware file from here that is tailored for your device and save it to your desktop. Also, the downgrade process will erase everything off your iOS device therefore backup everything in a safe and secure location. You can either use iCloud or iTunes. Whichever feels convenient.
1. Connect your iPhone or iPad to your computer using a Lightning cable.
2. Now launch iTunes.
3. Once your device is detected, you'll see it appear in the top left hand corner. It will appear as a tiny iPhone icon. Click on it.
4. You'll see more option on the right. Click on the 'Restore iPhone' button while holding down the left Option key (Mac) or the left Shift key (Windows).
5. Select the iOS 11.2.5 firmware file saved on your desktop and the downgrade iOS 11.2.6 will begin immediately.
If iOS 11.2.5 if still being signed by Apple, which it is at the time of writing, the firmware will downgrade smoothly. The entire process won't take more than a few minutes and you can start using your device when done immediately.
Quite frankly, I don't see any reason why anyone should downgrade from iOS 11.2.6 to iOS 11.2.5. The Telugu crash fix is extremely important and trolls have been posting the character on social media just to crash devices. If you really don't want to be a part of the annoyance, then please, stick with iOS 11.2.6.