#1
Its like..... I wrote some songs on Em scale(some melo death kinda stuff) ....But dont knw how to change scale of the sing in the middle of the song. U knw what i mean? Like putting a bridge which should not sound like songs other part and which should sound different from the rest of the parts of the song....

So now please tell me how to do it. Im using Em scale.

Thanks in advance...
#2
"Em Scale"...I assume you mean E Natural Minor?
You could always change scale into something like Harmonic/melodic minor and things like that. Or switch to the relative major key (though that isn't very melodeath I believe!).
Or switch to a different mode (E Phyrigian, Locrian, etc etc, the minor sounding ones).

Or just change the key of the song completely, but that's not always a great thing to do unless you can do it right, cos the song can end up sounding completely off.
#3
just write a riff in anther key that fits the flow of the song, if your in Em I'd try Bm