And how can I change from tabs into spaces or from the other way around?
The default Tab Type is “auto”. You can find this information in the Editor settings in the Settings View:
You can also set your default there.
Thanks, does change this automatically convert existing tabs into spaces and vice - versa?
How can I see whether I have tabs or spaces? In SublimeText tabs is shown as long line while space represented as dot. How can I get this in Atom?
You can turn on “Show Invisibles”, also in the Editor settings.