The program HDECAY determines the partial decay widths and branching ratios of the Higgs bosons within the Standard Model with three and four generations of fermions, including the case when the Higgs couplings are rescaled, a general two-Higgs doublet model where the Higgs sector is extended and incorporates five physical states and its most studied incarnation, the minimal supersymmetric Standard Model (MSSM) with real soft SUSY-breaking parameters. The program addresses all decay channels including the dominant higher-order effects such as radiative corrections and multi-body channels. Since the first launch of the program, more than twenty years ago, important aspects and new ingredients have been incorporated. In this update of the program description, some of the developments are summarized while others are discussed in some detail. Program Title: HDECAY Program Files doi:http://dx.doi.org/10.17632/3ggyvkjz95.1 Licensing provisions: GPLv2 Programming language: FORTRAN77 Journal reference of previous version: Comp. Phys. Comm. 108 (1998) 56-74. Does the new version supersede the previous version?: Yes Reasons for the new version: Major updates and extensions Summary of revisions: Since the first release of the original version of the program some bugs have been fixed, a number of improvements and new theoretical calculations have been implemented. The logbook of all modifications and the most recent version 6.52 of the program can be found on the web page http://tiger.web.psi.ch/hdecay/. Nature of problem: Decay widths and branching ratios for Higgs bosons decays in the Standard Model with three and four generations of fermions and rescaled couplings, a general two-Higgs doublet model and the Minimal Supersymmetric Standard Model are calculated numerically, including loop corrections according to the current theoretical knowledge. Solution method: One- and two-dimensional numerical integration of analytic formulae for Higgs boson decays into off-shell particles. All other decay widths are calculated analytically. The transition between off-shell and on-shell decays is performed by a linear interpolation.