Update old record

I have a requirement where I need to update the record in mart if the source table record has beedn updated. Right now if we dont get the attribute info for that particular record, I hard cord that as 'NULL'/'Unknown'. 

Once the source has been updated of got the attribute info , I should be able to update the corresponding record in Mart.

Here is the example:

next day, i got the updated info:

The dim1 has fk relationship with dim2 table

How to update the record in dim2key based on dim1 and source.

Thanks in advance.

ZK