Video help Needed

Hi.
Im hoping someone can help me out.

I recorded a video on xbox and I want to post it on the BL2 forum , but the person I was splitscreening with doesnt want their gamertag shown.

I downloaded the vid and used an editing app to block out his part but it wont upload to the forum , something about the wrong file type I think.

Is there a good app for converting file types , or better way to edit the clip?
I dont really know much about video stuff so any advice would be appreciated.

The video has to be embedded from another site. So, either xboxdvr (for the original recording) or youtube, vimeo, … The site doesn’t support local copies of videos.