Content creators are always tweaking the information of their videos (titles, tags, descriptions, and so on). In some cases, you may want to update this information on multiple videos at once—but editing each one individually can take quite a bit of time. YouTube now allows you to update certain information for multiple videos at once.
From your video manager, select the videos that you want to update by clicking the checkboxes to the left.
If you wish to edit all of your videos at once, click the top most checkbox, then choose Select all your videos.
When your videos are selected, click on the Actions button at the top, then choose Advanced to see all the information that can be edited. (If you chose all videos, the Advanced list will appear automatically.)
One reason you might want to bulk-update your videos is to add a new tag to all of them. For example, say you analyzed your traffic sources and discovered that viewers were finding your videos after searching for your full name. You might want to add your full name as a tag.
Selecting the Tags option will give you a few choices: Append, Set to, and Remove.
Append: Adds the new tag to the end of each video's current tags.
Set to: Replaces the current tags with the new ones.
Remove: Removes the entered tag(s) from the selected video(s).
In this case, Append will allow you to add your name as a tag to all your videos.
*IMPORTANT NOTE: As a general rule of thumb: If you are happy with a video's current performance, DON'T change anything. Changing metadata (like title, description, and/or tags) can result in lower views if YouTube has already found your video to be of value to viewers.