Open preferences, in macOS ?; or in Windows/Linux Ctrl + Alt + S.
Go to Editor -> Code Style -> Python, and if you want to follow PEP-8, choose Tab size: 4, Indent: 4, and Continuation indent: 8 as shown below:
Apply the changes, and click on OK.
Option 1: You can choose in the navigation bar: Edit -> Convert Indent -> To Spaces. (see image below)
Option 2: You can execute "To Spaces" action by running the Find Action shortcut: ??A on macOS or ctrl?A on Windows/Linux. Then type "To Spaces", and run the action as shown in the image below.