I need to update a cell with info from another row.

Use A to find the correct row inn C. Move the value D into cell B:

A B C D
1 x 1 123
3 x 2 234
5 x 3 345
..... 4 456
..... 5 567

becomes:

A B C D
1 123 1 123
3 345 2 234
5 567 3 345
..... 4 456
..... 5 567