While committing package to launchpad through quickly submit, version automatically jumped to current month, that is 12.07. I did try to delete all milestones and re-upload the package. But every time, launchpad is rejecting submission due to "Version older than that in the archive". How to fix this ?
You cannot upload a version older or equal to, a version you've already uploaded to a PPA. My advice would be to live with the mistake, and don't make the same mistake in the future. Be sure everything is right, before you upload something to a PPA.
You can use sbuild or pbuilder to run test builds locally in a chroot, to help ensure everything builds properly, before uploading.