According to Kesavananda Bharati (1973) case, the Supreme Court held that the Preamble is a part of the Constitution.
The court stated that the opinion tendered by it in the Berubari Union (1960) case was wrong, and held that the Preamble can be amended, subject to the condition that no amendment is done to the ‘basic features’.
In other words, the Court held that the basic elements or the fundamental features of the Constitution as contained in the Preamble cannot be altered by an amendment under Article 368.
The Preamble has been amended only once so far, in 1976, by the 42nd Constitutional Amendment Act, which added three new words – socialist, secular and integrity – to the Preamble. This amendment was held to be valid.
Further Reading :
To more about the Kesavananda Bharati Case, check the linked article.