MrBayes may be downloaded as a pre-compiled executable or in source form (recommended).
The most recent release version of MrBayes is 3.2.7a, released March 6, 2019. The 3.2.7a source code is available for compilation on Unix machines. Pre-compiled (provisional) executables are available for Windows (MrBayes-3.2.7-WIN.zip). These are, however, serial versions without the BEAGLE library. The serial version works well for smaller analyses but if you plan to run large analyses using many parallel chains, you should use the MPI version instead. Refer to the User Manual, and the INSTALL document on GitHub for help with installation of the program.
You can get access to older releases (from release 3.2.0 onwards), by browsing the releases directory on github.
You may also download cutting edge developer version of MrBayes from the Git repository hosted at GitHub. Note that you have to compile the code yourself. Read instructions in the INSTALL file in the source code repository for further instructions.