while i click on install updates,and after getting authentication a message is displaying that " it needs installation of untrusted packages". what am i supposed to do ??

Well that depends.

Open your terminal and run;
copy ALL error or warnings to gedit or your favorite text editor and then run;
and do the same.

Send both those, complete, here.

Then we will know what repo you are having trouble with.

1 members found this post helpful.

Usually you will have to say yes or no, if you want to continue the installation say yes, if you are worried about untrusted packages say no and it will abort the installation.

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Unless you have deliberately added any third party repositories to your system (and this includes those PPA repositories) you should be ok. If you are only using the standard Ubuntu repositories you have nothing to worry about.
If you want us to check your sources then simply post the contents of your
/etc/apt/sources.list file. This file lists all of your software sources where you can get updates from and install software from. Post the contents of the file in CODE tags (the # at the top of the post window) if you can please.
Also post the contents the contents of any files in your /etc/apt/sources.d/ directory.

And welcome to the LQ forums!

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thank u so much friendzz..