UPDATE based on if value exist in another table

Yes. You need to update using a join like this:

update t2
set t2.Another = 'ZZZ'
from table1 t1
join table2 t2 on t1.Number = t2.Number

You can use exists also.

Query

update t1
set t1.[Another] = 'ZZZ'
from [TableB] t1
where exists(
    select 1 from [TableA] t2
    where t1.[Number] = t2.[Number]
);

You can use directly SELECT FROM table1 and update into table2 :

UPDATE table2 SET Another = 'ZZZ' 
FROM table1 t1 WHERE t1.Number = table2.Number