You can edit custom-defined functions. You need at least Application Manager permissions to edit local (application-level) functions and Administrator permissions to edit global (server-level) functions.
To edit custom-defined functions:
From Enterprise View or a custom view, select an Essbase Server.
Right-click, and select Edit, and then Functions.
In the Custom Defined Function Manager window, select a function, and click Edit.
In the Edit Function dialog box, edit one or more fields.
Click OK.
Essbase updates the function.
create function (MaxL) in the Oracle Essbase Technical Reference