Check the SHA1 checksum of the file matches the filename - do not proceed if it doesn't.
Copy the zip archive to the ota folder you created.
Dial *#*#682#*#* from the phone app to open the offline OTA updater.
Select the zip archive from the dropdown, then tap "DIRECTLY APPLY OTA FROM SELECT" to begin the update process. This may take a few minutes and your phone will need to reboot to complete the update.