i cant view replays in game for some reason (they end up being incredibly laggy on my end, probably due to optimization) so i was wondering if there was a way to download the file, and there is but it's labeled as a REPLAY file and there doesn't seem to be a way to convert it to an mp4 that i can find. are there any programs that can help, or a method i can do?

look at main menu at replays and at the bottom should be an option for replays in folder than double click which one

There is no way to convert a replay file to MP4 as of yet no. If the ingame viewer is laggy for you your best bet is to send the replay files off to a friend and have them record them for you, or wait/hope the option to save replays as video files gets implemented eventually