Hi, two questions: how do you tap volume up and down at the same time?

And I'm trying to clear the cache using the suggestion on the HTC One site, however, when I go into settings then storage and go to content it shows me a bunch of headings with how many files are in there but the last category is Miscellaneous and mine has 4,127 files. When I go in there, there are 25 sub categories with different amounts of data being stored but it isn't clear if any of this is just cached files or things I shouldn't touch! How do I know what to delete? For example under .data it says 68.27MB. But what is included in that!?!?