Change drupal standard messages like "You have just used your one time login link"
Try the String Overrides module. If you have more strings to change.

Provides a quick and easy way to replace any text on the site.
Features

Easily replace anything that's passed through t()
Locale support, allowing you to override strings in any language
Ability to import/export *.po files, for easy migration from the
Locale module
Note that this is not a replacement to Locale as having thousands of
overrides can cause more pain then benefit. Use this only if you need
a few easy text changes.