Go to Setup > Company > Classifications > Subsidiaries
Adding Nexus to a subsidiary:
If the subsidiary has valid registration to collect tax from that nexus, then you can add that nexus under the subsidiary.
Removing Nexus from a subsidiary:
Before removing nexus from a subsidiary check if that subsidiary has associated transactions. If the subsidiary has transactions linked to that nexus then system restricts from removing and throws an alert. So you need to delete or void those transactions before removing nexus.
Deleting Tax Nexuses:
Go to Setup > Accounting > Taxes > Nexuses.
Before Deleting nexus check whether that nexus has associated transactions, If that nexus has associated transactions first delete or modify those transactions else system throws an alert and restricts from deleting.