This can be done in several ways. One way is to use join as described in earlier post, another could be to use correlated (or non-correlated) sub-queries. One simple example:
UPDATE MyTable
SET Field1 = (SELECT Something FROM AnotherTable WHERE MyTable.Field2 = AnotherTable.AnotherField)
WHERE ...
If you could write the requirements and perhaps an example, then the solutions could be targeted to your psecific problem.