how do i upgrade from 4.2 to 4.21?

i'm not seeing an actual set of instructions.. i have the open source version

Same as always: overwrite the files.

sorry if this is really basic, just never upgraded before.. just started on 4.2

do i overwrite both cgi-bin and mt-static?

is there nothing in there that needs to be saved, or do all those files remain untouched for the life of the blog?

Yes just upload the mt and mt-static folders and overwrite everything, that is what I did at the weekend.

The truly paranoid can take an sql export of their database using phpymadmin or similar and a backup of mt-config.cgi