Ah okay you have probably removed some system files and that is what is causing the errors when updating since you are rooted. Yes you would lose root if you flashed the ftf file and from looking at a few other posts you can't root .257 yet.
Search for flashtools and you will find numerous guides on here. After you have it all installed and select the firmware to flash just uncheck the wipe data check box that way it will keep your apps and settings.
The ftf file is in my signature. If you need any more help or have other questions let me know.
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Thanks button if I helped.