One of my answers was deleted due to an alleged plagiarism. I tried to change the answer by adding more information and rewording and flagged it to be undeleted but the flag was declined. How can I have it undeleted?
Besides the fact that the edits don't actually address most of the actual plagiarism that the original post was deleted for (compare, for example, http://islamquest.net/en/archive/question/fa2018), undeleting posts that were plagiarised is not standard practice here. As Shog9 mentioned last time he weighed in on the rampant plagiarism on this site (emphasis mine):
Allow authors to re-post answers when the new answer corrects the problems identified in the old one. Refrain from undeleting answers that were originally plagiarized - this just rewards trickery.
Your original post was by all appearances heavily plagiarised; every single action (including votes) taken on it was based entirely on the work of someone else rather than your own. Even if your changes did address the plagiarism, such changes would render the new post so fundamentally different from the original that every single vote it had already attracted would become wholly irrelevant.
There is no reason to undelete this post. Just post a new answer — one that is actually original content based on your own knowledge and expertise rather than just regurgitated from somewhere else (i.e. plagiarism) — and let the system work as intended.