Make sure there are no errors displayed on the screen. Errors are displayed in red near to the are or field that is invalid. Once all errors are resolved, the Save button should be available.
However, you may notice that the Save button does not display and you cannot interact with editable fields. This is likely because the application has been disabled for editing by a System Administrator. In this case, you will not be able to make changes in the application until editing is enabled again by a System Administrator.