Default Help needed regarding Update Error

Hi there. First of all I am sorry to create a new topic because I have already spent 2 days of searching the forum about my problem but couldn't quite figured it out rightly.
I have Samsung Galaxy S4 I9505. Its operating system is 4.3. When I try to update it via OTA, it says "Your software has been modified. Software updates are not available. "
But I don't know if I have done any modifications in my software.
Kindly help me in this regard.


Sent from my GT-I9505 using xda app-developers app
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