they are exist in different location in .impex
files (importer files) , that you initialize the store for the first time , those .impex
files got imported to your database
example of one location :
ext-template\yacceleratorinitialdata\resources\yacceleratorinitialdata\import\common\user-groups.impex
as there are multiple starter stores that comes with hybris (accelerator, telcoaccelerator, and powershop b2b) I suggest you to search as text for username or password in all files extention = .impex then change for files that belong to your store .
i did a quick search , not only in .impex file , the hmc admin/nimda seems to changes from
/bin/platform/project.properties
# Login and password for the automatic logging into the hMC
hmc.default.login=admin
hmc.default.password=nimda
hope that helped you .
Thanks