To toggle between tabs and spaces:
On the main menu, choose Edit → Convert Indents, and then choose To Spaces or To Tabs respectively. Read more about Changing Indentation.
You can quickly do the same with Navigating to Action menu by pressing Ctrl+Shift+A and type convert indents in the field:
If you want to set spaces indents by default, go to Settings → Editor → Code Style → Java, make sure the Use tab character checkbox is unchecked and set the Indent value:
