UPDATE Column(s) one table from a table in another database

Select Database Where update needs
Or Write USE Database
If Database selected then no need to write USE DB1

USE DB1 --(optional)
UPDATE DB1.dbo.MyTable
SET
  Field1 = d2.Field1,
  Field2 = d2.Field2,
  Field3 = d2.Field3
FROM DB2.dbo.MyTable d2
WHERE d2.MyKey = MyTable.MyKey