Currently it is not possible to delete or change custom defined schema extensions with the available tools.
You can change the data type of a column with a SQL statement such as the following:
Please note the information in the script(s) provided is known to work successfully; however, they have not been officially tested by One Identity Quality Control.
If any of these instructions are changed and/or incorrectly used, intentionally or unintentionally, this solution becomes unsupported by One Identity Support and Development.
One Identity Support and Development recommend always making a backup of the current database prior to execution of any script(s) that may modify it.
alter table SE_CustomTable1
alter column SE_Column2 nvarchar(128)
But depending on the source type and the current content of the column you could get errors, unless only the length is being changed.
After the execution of the statement you should check it in the Designer -> Database schema -> open the column -> tab "More" -> field "Maximum size in DB".
Enhancement request 20845 has been submitted to Development for consideration in a future version of Identity Manager to allow for this functionality.