Youtube makes it really easy to create accessible videos with transcriptions. As you have noticed, however, they aren't always perfect. Luckily it is very easy to fix, and they have a great article on how to do it. To summarize:
- Go to the video you want to change in the video manager, click 'Edit', and then 'Subtitles and CC'.
- Here you can edit any part of the captions you would like. Then hit save.
For more detail information on youtube captions, you can visit their youtube help question here.