find . -name "*.yaml" | xargs perl -p -i -e "s/foo/buzz/g"
find . -type f -name "*.yaml" -print0 | xargs -0 sed -i '' -e 's/foo/bar/g'

I recommend installing and using repren if you need a simpler solution: https://github.com/jlevy/repren

Reference:

• https://stackoverflow.com/a/8575384
• https://superuser.com/questions/428493/how-can-i-do-a-recursive-find-and-replace-from-the-command-line