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The best custom ROM for Test Device S25 Ultra depends on your priorities — stability, battery life, or the latest Android features. RomX lists all actively maintained ROMs for Test Device S25 Ultra with real user ratings, changelogs, and install guides so you can compare them directly.
Enable Developer Options (tap "Build number" 7 times in Settings > About phone), turn on "OEM unlocking" and "USB debugging", then reboot into fastboot mode and run the unlock command for your brand (e.g. `fastboot flashing unlock` or `fastboot oem unlock`, depending on the manufacturer). This wipes all data on Test Device S25 Ultra, so back up first. Exact steps vary slightly by brand — check the install guide on each ROM's page for Test Device S25 Ultra.
Unlock the bootloader first, download the correct TWRP (or OrangeFox) recovery image for Test Device S25 Ultra, then flash it from fastboot with `fastboot flash recovery twrp.img` (or boot it temporarily with `fastboot boot twrp.img` to test before flashing permanently). Never use a recovery image built for a different device — it can brick your phone.
Installing a custom ROM is generally safe if you follow the correct steps for your device and back up your data first, but it can void your warranty and carries a small risk of a bricked device if the wrong build is flashed. Always verify the ROM's checksum and read the known-issues list before installing.
In most countries, unlocking the bootloader and flashing a custom ROM does void the manufacturer's warranty, since it modifies the stock software and trips the device's tamper flag. Some manufacturers restore warranty eligibility once you flash back to an official stock ROM and relock the bootloader — check your brand's specific policy.
Download the official firmware package for Test Device S25 Ultra from the manufacturer, then flash it using the brand's official flashing tool (e.g. Mi Flash Tool, Odin, or fastboot, depending on the manufacturer) in fastboot/download mode. This restores the original software; you can also relock the bootloader afterward if you want to reinstate warranty eligibility.