Follow along with the video below to see how to install our site as a web app on your home screen.
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once you enable them in menu>settings>language&keyboards all you need to do is open text message (or email) and long press in the text input box and you will get the 'input method' option. It is there that you can switch between them
On the HTC One if you have more than one keyboard enabled there will be a keyboard chooser in the notifications any time you have a keyboard active.

I can swap between the stock keyboard and Swiftkey (the two I currently have installed) using the notification switch. And I just installed Handcent temporarily to check that it works when using that app.