I assume that your question is about Oracle-OLAP dimension attribute (and not Essbase).
You will have to come up with a custom front-end (web or windows) where you will enter (or change) the attribute value. Then behind the scenes, you will call DBMS_CUBE.BUILD procedure to update the attribute value for that dimension member.
Here is an example:
Dimension = 'ACC', Hierarchy = 'ACCHIER', Dimension Member = 'CM1' and we want to update an attribute value called "LONG_DESCRIPTION" for this member 'CM1'.
begin dbms_cube.build(q'! "ACC" USING ( SET "ACC"."LONG_DESCRIPTION"["ACC" = 'CM1'] = CAST('Custom Member 1' AS VARCHAR2), MERGE INTO HIERARCHIES ("ACC"."ACCHIER") VALUES ('CM1', 'ALL_ACC', NULL), COMPILE)!'); end;