It's rather a long process to describe here, but essentially you have to find the firmware somewhere online - possibly the manufacturer's website - download it to a PC, and flash it to the phone in whatever way you can. The best place to start is the manufacturer's website, for both firmware and flashing tools, but don't be surprised if you have to go looking elsewhere.
It would be a good idea to read about these things in detail before attempting anything, but we'll help if there are any steps about which you are uncertain. Someone here may even help you with all the steps, but I recommend registering as a member first.