Youtube Bumper - Editing Videos

Hi all,
This is an ad that I inserted!
So I have a YouTube bumper, an intro and outro, and I was wondering what the best, easiest and quickest way is to edit my videos? Also, is there any way to edit videos already uploaded to YouTube without uploading again?

Most all videos I take with my phone or iPad, not sure if this matters? Any help would be greatly appreciated.

Thanks,
Jo

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