If you cancel a ticket, the seat reservation automatically cancels as well - no worries.
Check DB's info on canceling / changing seat reservations, and follow the link to change them:
https://www.bahn.com/en/view/booking-information/booking/exchanges-refunds.shtml?dbkanal_007=L04_S02_D002_KIN0059_FOO TER-BUCHUNGSINFORMATION-UMTAUSCH-ERSTATTUNG_LZ01