I want to change "Group" word to "project" in organic group module, I am using drupal 6.x with postgre.

You can do this various ways. One way is to use hook_alter() to change the text, but this isn't a great method for a few reasons. One reason is that it sometimes breaks button functionality here and there. The other is that it uses a fair bit of extra code, and you have to write more code.

Another better way to do it is to install another language on your system. Make a custom language, and call it whatever you want. The way translations work is that they look for a translated string, and if they don't find one, they show the original text. So what you do then is 'translate' any string you want into another string. So you search for everything that has 'group' in it, and you change that to 'project'. This method will not break any buttons.

The last method is to use the string override module. This essentially lets you do the same thing as above, but you don't need to add a new language. This also won't break any buttons.