The Ultimate Guide to Converting WebM to MP4 (EASY & FREE)

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I recently bought an app for my android device with which to record home movies using the device's camera and utilising various filters and 'live' effects.
It's a cool application and does everything i want it to, but the devs decided not to buy the rights to any file format that included royalties. Therefor, my home movies are saved only as WebM files that are accessable soley through the web (firefox html).
This means that i have absolutely no way of PC editing or using my own movies taken from my own device! - which is pretty frustrating as i am eager to make some original youtube/vimeo films very soon.

This is a bit sneaky and dishonest by the app devs as they should tell people this before buying the program, but the program itself is perfect (and pretty unique) for my movie-making needs.
So, how can i convert my own personal recordings from a WebM to a saveable and editable file format on my Windows PC, and is there a safe video converter out there that i can use for this purpose?
(Preferably low cost or free, but above all, safe).

Thanks,
KJ

Видео The Ultimate Guide to Converting WebM to MP4 (EASY & FREE) канала Kulvir Ageti