Create/Update table help

I'll try to explain this the best i can.

I have 2 tables one a customer table, which holds customer names and the default address for that customer. The 2nd table has the defualt delivery address, plus other delivery addresses for the customer (so maybe there work address or home address).

The data in the development databases are scrambled so all the customers addresses arnt real, the problem i have is i would like the defualt address to be the same in table a and table b.

My original thought was to update the default address in table 2 by doing a simple update from table 1 but this took way too long (there are around 10million rows).

So my question what is the best way to get the address from table a to update the default address in table b. Table B has a default_address_id column which specifies that this is the address shown in table A.

Hope this makes sence, if not i will try to clarify if you are able to help.