The fundamental laws shall be modified or amended by means of two decisions of identical wording, and the second decision may not be taken before a general election has taken place and the newly-elected Parliament has convened (cf. The Instrument of Government Chapter 8 Art. 15).
Besides the above: The Riksdag Act (The Parliament Ordinance; Act 1974:153 - in Swedish only), which contains more details regulations concerning the Riksdag and how it works. The ordinance constitutes a intermediate position between the Constitution and the ordinary acts, and can more easily be modified.