Trying To Update Field Based On Other Table

I have the following situation.
Table 1:
------------------------
Primary Key
Name
Secondary key

Table 2:
------------------------
Primary Key
Age
Secondary Key

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This is just an example. The first table is totally populated. The second table does not have the secondary key assigned, but otherwise is populated. In additon, some of the records in Table 2 do not have a matching entry in table 2.
I would like to be able to update table2, populating the Secondary Key field in order to allow me to delete the Primary Key fields. The Primary key was assigned by somone else who created the db, and makes no logical sense, i'm trying to replace it with a key that can be used more easily. Any help would be greatly appreciated.

Kevin S. Jones