update bios

hi !!
ok,i am trying to update my bios but before installing it in i have to go to ms dos mode.i created the ms dos startup disk.then i restart when the ms dos mode prompt ,(put my bios disk in )i type aflash.exe but it say bad command ,i am not sure what the problem was but i know my start up disk missing alot of things , i have done before but i could figure out the web site that could help me step bu step .Any could help me ?would be appreciate.