As you can see from Section 2.7 of Multiwfn manual, user-defined functions 1 and 2 correspond to alpha and beta densities, respectively. Therefore, assume that you want to generate cube of alpha density, you just need to set "iuserfunc" in settings.ini to 1. After booting up Multiwfn and load wavefunction file, enter main function 5. Then when Multiwfn asks you to choose the real space function, select "100 User-defined function", then alpha density will be calculated, and you can export cube file in post-process menu.

It is quite easy. Subfunction 26 of main function 6 can manually modify occupation numbers of various orbitals. In this function, input indices of beta orbitals first, and then input 0 to set their occupancy to zero, then return to main menu. After that, if you calculate electron density as usual, then the result will correspond to alpha density only. Main function 5 can compute grid data of a selected real space function and export a cube file.

Note that the index ranges of alpha and beta orbitals are automatically shown on screen after loading an unrestrricted wavefunction.

Best regards,

Tian