GNU Emacs Manual
S.3 Tabs vs. Spaces
Emacs normally uses both tabs and spaces to indent lines. If you
prefer, all indentation can be made from spaces only. To request
nil. This is a per-buffer
variable, so altering the variable affects only the current buffer,
but there is a default value which you can change as well.
See section AD.2.4 Local Variables.
There are also commands to convert tabs to spaces or vice versa, always
preserving the columns of all nonblank text. M-x tabify scans the
region for sequences of spaces, and converts sequences of at least three
spaces to tabs if that can be done without changing indentation. M-x
untabify changes all tabs in the region to appropriate numbers of spaces.