How to convert indentation to space in Vim ?

I typed following command in vim to show white space characters, such as eol, tab, and space.
:set listchars=eol:$,nbsp:_,tab:>-,trail:~,extends:>,precedes:<

The file was cloned from git repository.  Indentation is setup for tabs.  How do I convert indentation to space?

 50 addtask listtasks$
 51 do_listtasks[nostamp] = "1"$
 52 python do_listtasks() {$
 53 >-import sys$
 54   print('Hello World')$
 55 >-for e in bb.data.keys(d):$
 56 >->-if d.getVarFlag(e, 'task', False):$
 57 >->->-bb.plain("%s" % e)$
 58 }$