First, order a Russian keyboard for your phone. You can either find it in the shops and service centers in your city, or enjoy using the Internet. If you want to spend a minimum of effort, you can order me at any service center, but if you want to save money, you can do this yourself.
If the package is present in the USB cord and the drivers, connect the phone to the computer and pre-install the necessary software. If this package is missing, download drivers from the official Samsung website, or from any other source. Also take care of downloading the Russified firmware and software for the implementation of flashing the phone. Depending on your phone model and operating system you are using, can change the principle of firmware, so you can also download the user manual for the software.
Synchronize your phone with your computer, make sure that the software "sees" the phone. Then use the program for flashing. Follow the directions carefully and be sure to save the original firmware. In case of any problems that you cannot resolve, please contact the service center.