Write text to a file in Azure storage

I have a text file uploaded in my Azure storage account.Now, in my worker role , what i need to do is every time it is run, it fetches some content from Database, and that content must be written in the Uploaded text file, specifically , each time the content of Text file should be overwritten with some new content.

Here, they have given a way to upload a text file to your storage and also delete a file.But i don't want to do that, need to just MODIFY the already present text file each time.

I'm assuming you're referring to storing a file in a Windows Azure blob. If that's the case: A blob isn't a file system; it's just a place to store data (and the notion of a file is a bit artificial - it's just... a blob stored in a bunch of blocks).

To modify the file, you would need to download it and save it to local disk, modify it (again, on local disk), then do an upload. A few thoughts on this:

For this purpose, you should allocate a local disk within your worker role's configuration. This disk will be a logical disk, created on a local physical disk within the machine your vm is running on. In other words, it'll be attached storage and perfect for this type of use.
The bandwidth between your vm instance and storage is ~~100Mbps per core. So, grabbing a 10MB file, while on a Small instance, would take maybe a second. On an XL, maybe around a tenth of a second.~~ really fast, and varies with VM series (A, D, G) and size.
Because your file is in blob storage, if you felt so inclined to do so (or had the need for this), you could take a snapshot prior to uploading an updated version. Snapshots are like link-lists to your stored data blocks. And there's no cost to snapshots until, one day, you make a change to existing data (and now you'd have blocks representing both old and new data). An excellent way to preserve versions of a blob on a blob-by-blob basis (and it's trivial to delete snapshots).

Just to make sure this download/modify/upload pattern is clear, here's a very simple example (I just typed this up quickly in Visual Studio but haven't tested it. Just trying to illustrate the point):

        // initial setup
        var acct = CloudStorageAccount.Parse(CloudConfigurationManager.GetSetting("StorageConnectionString"));
        var client = acct.CreateCloudBlobClient();

        // what you'd call each time you need to update a file stored in a blob
        var blob = client.GetContainerReference("mycontainer").GetBlockBlobReference("myfile.txt");
        using (var fileStream = System.IO.File.OpenWrite(@"path\myfile.txt"))
        {
            blob.DownloadToStream(fileStream);
        }

        // ... modify file...

        // upload modified file
        using (var fileStream = System.IO.File.OpenWrite(@"path\myfile.txt"))
        {
            blob.UploadFromStream(fileStream);
        }