Update all data in a table without affecting ID(PK)
I am not fully clear regarding where you get the new CustomerCredit value but an SQL update should never change ID unless you explicitly tell it to.

UPDATE [table] SET CustomerCredit = 100.0 WHERE ID = [customer-id-to-update]

or setting all records to the same value, just omit the WHERE

UPDATE [table] SET CustomerCredit = 100.0