Push command not working says permission denied

I rooted my micromax p275 and wanted to change its font by replacing /system/fonts/Roboto-Regular.ttf with my desired font but i accidentally replaced it with Robot.Regular.ttf. Now it doesn't starts. I want to push another font file in /system/fonts with the correct name (Roboto-Regular.ttf) but it doesn't works saying permission denied. I have tried su command but it just reprints the same commands i type and press enter and thus doing nothing. Any way out friends. I would be greatly helped by you. Every single hint is wanted.