ALTER TABLE not possible

Hi everyone,

I've implemented Replication in my both instances Sql2k12.

Publication type is Snapshot.

Now, trying to modify some columns or constraint for any table I get this message (similar in each case):

'EmployeeLogon (Acc)' table
- Unable to delete index 'PK_EmployeeElectronicJournal'.  
Cannot alter the table 'EmployeeLogon' because it is being published for replication.
Could not drop constraint. See previous errors.

Let me know which would be the upcoming steps in order to allow ALTER COLUMN/CONSTRAINT in my schema tables.

Thanks for your time