I use KeepVid video converter to convert my files into different formats without much effort. This software can be used even in an online mode without downloading it to the local disk. I was impressed by its awesome features. You can convert the files from one format to another easily at a faster rate.
There are many online video converters now, you can find some relevant for you with google. Also you can install application on your PC. I recommend you to search guides exactly for your tasks. For example here is a guide "How to convert MP4 to FLV" - www.videoconverter.com/how-to-convert-mp4-to-flv.html