How do I update Android 4.3 firmware on Micromax a121?

i wish to re-install android 4.3 on my micromax a121. while the software update option says that the software is uptodate, my phone takes a long time to come alive

Welcome to the forums. Have you received any kind of update? If so, have you done a factory reset?

If the update option says that the software is up to date, it means that there's no later version available for your phone from your carrier - and that's the only place it's going to come from. Besides, updating won't make the phone start up any faster, if anything putting 4.3 on a phone designed for 4.0 will make it slower.