How can I use PyPy as standard/default interpreter in PyCharm 4.5.2? Everything runs under Ubunutu 14.10 and pypy is already installed.
You configure this under the settings for the project. This is directly covered by the official documentation.
In case that link fails to open, go to the 'settings', then the 'project pane', and then select your desired interpreter from the drop down. If it's not listed in that window, click the cog, then add 'add local', and navigate to the interpreter you desire.