Xperia Arc will not Update

[#1]

My Xperia Arc is tell me i have a update, but when i try to update it, i get the message update failed phone contains modified software.

I have bought this phone secondhand, so i don't know what the guy before me has done, also i live in Australia but i am fairly sure the phone has come from Europe.

Thanks in advance for any help you can give me.

Regards

          Mac

[Nessa]

If the phone software has been tampered with, our Update Service application will not be able to install the latest software version.

[#3]

So theirs nothing i can do? will the phone continue to work with out updating