The OS does not match the system requirements of the smartphone.
Where can I find the firmware?
On the phone manufacturer's website.
On specialized resources where users post custom or official OS.
What should be done before installing the firmware?
Back up your phone's user data.
Insert SD card. An SD card is needed to write firmware to it.
Find information about your smartphone model.
Fully charge your phone. If the battery runs out during boot, the device will no longer turn on.
Find and download the archive with Firmware. Place it on the SD card.
Installing TWRP Recovery
Download the Official TWRP App from the Play Market. And install it.
When you start the application for the first time, you must agree to install the firmware, as well as agree to grant the application Superuser rights and click the 'OK' button.
Going to the next screen, select the item 'TWRP FLASH' and provide the application with root-rights.
On the main screen of the application, select 'Select Device', and find your phone model.
After selecting a device, the program redirects the user to a web page to download the corresponding image file of the modified recovery environment. Download the suggested *.img file.
After downloading the image file, go to the main screen of the Official TWRP App and press the 'Select a file to flash' button. Select the file downloaded in the previous step.
Click the 'FLASH TO RECOVERY' button and confirm your choice, click 'OK' in the question window.
When the message 'Flash Completed Succsessfuly!' appears. Click 'OK'. The TWRP installation procedure is now complete.
Copy the required files to the SD card. Using a PC or laptop card reader, it will be much faster.
Insert a memory card into the phone.
To reboot into recovery, you need to use a special item in the Official TWRP App menu, accessible by pressing the button with three stripes in the upper left corner of the main screen of the application. We open the menu, select the 'Reboot' item, and then click on the 'REBOOT RECOVERY' button. The phone will reboot into the recovery environment automatically.
Firmware via TWRP
Before flashing, you need to delete all user data from the phone, this will avoid many problems. press 'WIPE' on the home screen.
Now you can start flashing. Click the 'Install' button.
The file selection screen appears. At the very top there is a 'Storage' button for selecting an SD card.
Select the location where you copied the files..
Select the firmware file and click on it. A screen opens with a warning about possible negative consequences, as well as the item 'Zip signature verification'. This item should be checked by placing a cross in the check-box, which will avoid using damaged files when writing to the phone's memory sections.
The procedure for flashing the phone will begin, accompanied by the appearance of inscriptions in the log field and the movement of the progress bar.
The completion of the firmware installation procedure is indicated by the inscription 'Successful'.