So you have your favourite tabbing convention, but you've ended up with some files in some other convention, for example tabs instead of 4 spaces. If you use Vim and already have it set up to your liking, the :retab command will replace indents in the current buffer with those matching your convention. It's a bit tedious to do this manually for each file, so if you trust Vim not to make a mistake (or you're using version control) you can use a bit of scripting-fu to make it easier:

for F in *.{c,h}pp ; do vim -c ":retab" -c ":wq" "$F" ; done