With iOS 26.n for i/Pad/OS and macOS 26.n for Macs, the Liquid Glass design ethos is baked into the operating system and application design. You cannot update/upgrade to these operating systems and opt out of that design effect.
You cannot uninstall i/Pad/OS 26.n or revert to an older operating system on these platforms. Provided you have a current Time Machine backup of macOS Sonoma, you can upgrade to macOS Sequoia 15.n which does not feature the Liquid Glass design. That process would entail you downloading macOS Sequoia per these instructions:
How to download and install macOS - Apple Support
and cancelling at the prompt to install it. Then you need to make a bootable 16 GB USB stick containing the Sequoia operating system following these instructions:
Create a bootable installer for macOS - Apple Support
You want to perform a last Sonoma Time Machine backup and then eject that Time Machine drive, label it with the date/time and version of macOS (e.g. Sonoma 14.7.2) and set it aside. In System Settings > General > Time Machine panel, remove [-] that Time Machine drive entry.
Now, with a new Time Machine drive whose capacity is 2 - 3x that of your internal drive, use Disk Utility to format the entire drive as APFS with a noteworthy Time Machine drive name, and then eject it.
Insert that bootable USB stick that you made and then shut down your Mac. And then do the following:
- Press and hold the power button until the System Volume and Options button appears.
- Select that mounted USB stick to boot into the Sequoia installer
- During the initial setup, you will be asked if you want to restore from a Time Machine backup. This is where you plug in that Sonoma Time Machine drive where it will transfer your user files and settings to Sequoia.
- If this does not work, you can always use Migration Assistant later with that Time Machine drive to achieve the same result.
- Finish the Sequoia installation.
- If the Migration of data from the Sonoma Time Machine drive already occurred during the initial setup phase, then eject that drive. If not, launch the Migration Assistant application and use that Sonoma Time Machine drive to transfer your user data and setttings into Sequoia.
- Eject the Sonoma Time Machine drive, and connect the new Sequoia Time Machine Drive.
- Visit System Settings > General > TIme Machine and add [+] the new Sequoia Time Machine drive with an hourly backup schedule. If there are other mounted drives you do not want backed up, add them in the Options panel.
- Perform a first (full) backup of macOS Sequoia to the new Time Machine drive. Check System Settings > General > Software Update for newer Sequoia updates. You will likely see Tahoe at this point. Further down on that panel will be Sequoia updates as they become available.
Understand that you are freezing your macOS operating system at Sequoia and the evolving trend for third-party application developers (including Microsoft) is to support the operating systems that Apple supports. Currently, that is the current operating system release, and the previous two releases. Thus, in the Fall of 2027, Apple will cease software updates to Sequoia.
I am writing this on macOS Tahoe 26.1 and although I prefer the pre-Liquid Glass appearance, I simply have adapted and the operating system remains rock solid for me.