I had downloaded a ".iso" image of the ubuntu website and for some official work i need to convert this to ".img" format. i see a lot of windows software's available but how do i do this conversion in linux?
Actually there isn't any difference between iso's and img's apart the extension. Look at this link.
So you have to just change the extension.
To me an iso file is an image (img) file with the iso9660 file system. In other words, iso files are a subset of image files. As a matter of fact, most iso files are synthetic images, not created by making images of drives, but typically made in order to create boot drives in DVD disks, USB pendrives or memory cards.
So I agree with the answers, that you can replace the extension iso with img.
mv file.iso file.img
In order to make an img file accepted by some cloning tool that wants iso files, you could do it the other way too. But in principle that may not be quite correct, because an img file might be an image of a drive or partition, that does not contain an iso 9660 file system (and it might not be an image of a bootable drive). So it could make people confused.