You can use the Domain Aggregate function DSum to do what you require. For example:
Table1 with two fields: Key, Value
Table2 with two fields: Key, Value
Objective: Update the value fields in Table2 with the sum of the Value fields for matching Key in Table1.
On the premise that Key is a Text field and Value is a Number field, here is the query:
UPDATE Table2 SET Table2.Value=
DSum("[Value]","Table1","[Key]='" & Table2.Key & "'");