How to Fix: I can install the newest version of android, but after rebooting the phone,

The issue is caused by the phone's system restore feature, which reinstalls all files and apps to a previous version of Android (in this case, Android 15) after a reboot. This is likely due to the phone's manufacturer or carrier setting up the device with a specific version of Android.

🛠️ Step-by-Step Verified Fix:

1. Step 1: Disable System Restore: Go to Settings > System > Advanced > Reset options and toggle off the 'Reset device' option. This will disable system restore, allowing you to install the latest version of Android.
2. Step 2: Perform a Factory Data Reset: Go to Settings > System > Advanced > Reset options and select 'Factory data reset'. This will erase all data on the phone, but it will also prevent system restore from reinstalling older versions of Android.
3. Step 3: Install the Latest Version of Android Manually: Download the latest version of Android from a trusted source (such as the official Google website) and follow the installation instructions. This may require you to sideload the update, but it will ensure that you install the latest version of Android.