You can go back into the any of the videos you have loaded on your YouTube channel after you have gone through the upload process.  You may have not added them at the time of upload or you want to make changes to them.

1. Log into your YouTube account

2. Hit the down arrow next to your username to reveal your "My Account" menu  and select "Videos"

3. Locate the video you want to make changes to and hit the "Edit" button

4. You will then be presented with a screen very similar to the one when you initially uploaded your video where you can make the required changes to the description and tags you have assigned to the video.

REMEMBER - The words you assign to your video is what search engines see when they index your material, be descriptive and choose terms that are relevant to the content of your video.