im having a dimension table and a fact table

dim table:

reg_id  reg  City
1        r1    c1
2        r2    c2
3        r3    c3
4        r4    c4
5        r5    c5

fact table

customer  reg  city  reg_id
12         r1    c1
34         r1    c1
45         r4    c4
56         r5    c5
78         r5    c5

now i want to update the region id in the fact table based on the dimension table