First I'll double check that there are mixed tabs and spaces by highlighting the tabs.
If I see some whitespace highlighted and some not highlighted I'll look at the indentation and figure out how many spaces each tab should be, then do a replace. This will perform a replace on the entire file (%s) for all tabs (\t) into four spaces (\ \ \ \ ), the g at the end will allow the match to be done multiple times per line:
:%s/\t/\ \ \ \ /g
If your tab expand matches the file, you can also use the vim built-in retab: