Delete a Video Someone Else Uploaded to YouTube
"How can I delete a video if someone else uploaded it to YouTube?"
If someone else uploaded a video to YouTube, there is no way you can directly delete it. Only someone with access to the username and password for the YouTube account that uploaded the video can delete the video. If the video infringes on your copyright, if someone included you in their video without your permission, or if the video you want deleted violates the YouTube terms of service in some way, there are a few other actions you may be able to take.
Flag a YouTube video
One way to influence YouTube to remove a video is to point out violations of the YouTube Community Guidelines by flagging a video. The option to Flag a video is under every video on YouTube. You simply click the Flag icon and choose the reason for flagging the video. You do need to be signed in with your YouTube account to Flag a video and your reason for flagging the video needs to be valid or you run the risk of getting your account banned from YouTube, instead of having YouTube remove the video. If you have a legitimate complaint about the video that may also be shared by others you know, you can certainly let your friends and coworkers know about the situation so they can flag the video too.
Reporting Copyright Infringement
If a video infringes on your copyright, YouTube provides clear instructions on how to report copyright infringement. I have personally used this feature in the past to report someone else uploading my videos to their account. YouTube took action and removed the videos fairly quickly. Depending on how frequently you upload videos, YouTube now offers a way for you to claim your videos. In the event that someone else also uploads the same video, YouTube detects this and can either show ads you get paid for against the upload or force the offending uploader to delete the video.
What If I appear in a Video Without Permission
YouTube is less clear on how to deal with videos where you appear in the video and someone else uploaded it. If your appearance is the result of being in a public place, like at a park or walking on a public sidewalk, you are probably out of luck. If you were drunk at a party and said something stupid into someone's video camera, your best bet is to contact the person who posted the video and ask them nicely to take it down. If you can't get the person to remove the video by asking nicely, this may be an instance where consulting with legal counsel will serve you better than looking for free advice online.