Replace tabs with spaces in Emacs

In fact this is just a re-post of Julien Palards blog entryEmacs: replace tabs with spaces. I just want to keep it for my personal memory:

When you want to replace tab with spaces or vice versa don’t use M-% (query-replace) but M-x tabify or M-x untabify. They work on the current selection so if you want it to be applied to a whole buffer, try C-x h (mark-whole-buffer) before to select the whole buffer.