Instead of SEQUENCE like Oracle, Sybase has something equivalent, called IDENTIY.
By default, Adaptive Server begins numbering rows with the value 1, and continues numbering rows consecutively as they are added. Some activities, such as manual insertions, deletions, or transaction rollbacks, and server shutdowns or failures, can create gaps in IDENTITY column values. Adaptive Server provides several methods of controlling identity gaps described in “Managing identity gaps in tables”.
update GCOMElectronicOrder set TransactionID = 30000 where TransactionID = 21503