Assuming all you need to do is change the header information, then there are several free programs that might do the trick.
All of them are likely to use libtiff (http://www.remotesensing.org/libtiff) to provide TIFF support.
The most likely candidate would be ImageMagick (http://www.imagemagick.org/script/index.php)
DevIL (http://openil.sourceforge.net/docs/index.php) is image library.

Providing you understand the format your using (and it does store PPI information), you could as a last resort just create something to rewrite part of the header.