Remove background hiss from video

I have just shot a 33 minute video for YouTube on my iPhone. Unfortunately there is a load of hiss in the background (I didn't use a proper microphone) and I need to get rid of this hiss as soon as possible since I need to post the video
The only problem(s) is/are that the video is on my phone, it is a big file, and I have no idea how I could make this available to you to work on.
I will be very grateful if you can help.
Best wishes
Michael