I doubt you are truly looking for that.
Transcoding costs a lot of resources (cpu power, disk space) and takes a lot of time. If there were such a button, you wouldn't be able to use your computer for the coming month or longer, depending on the size of your library and the number of devices you want to view the media on.
Because of this PMS takes a different approach. Using lazy evaluation it only transcodes on the fly when it is really needed.
This is a good thing. Learn to love it.
In special cases transcoding in advance can come in handy, and there are other tools that help you to manually transcode files.
PMS was specifically designed to avoid manual transcoding.