how do i find and replace in dng

Unfortunately there's not a way to do that across artifacts UNLESS they are in a module (that would be a good enhancement request if you want to open one). 
Within a module, there is a Find/Go To button at the far right of the column headings (the magnifying glass icon).  That will let you do a find/replace across all the artifacts in the module. 
Depending on how many artifacts you have, it should be fairly easy to add them to a module just for this purpose.