Shalini, I finally resolved the issue. So, the issue is this:
For the marketplace to treat your uploaded asset as an update to an existing asset, it needs to have a properties.json file to look for the asset version, itemGuid, preview, and a couple of other important things, so that it can compare the newly uploaded package to the already-uploaded assets.
When we develop a project locally, there is no file such as properties.json in the project directory. There is a file named projectProperties.json, but that does not contain any of the required information for the marketplace. So, follow these steps:
a. Go to the marketplace link for the asset to be updated
b. Download the code, & spot the "properties.json" file
c. Update the 'assetVersion' to a higher one
c. Copy it and paste it into your local project's directory
d. Then export it & upload it on the marketplace
I hope this will help many users 