When you change the title text of the portal tenant 'Layer {tenantname} API Developer Portal" to something else its changed for a few seconds but after refreshing the change is are automatically rolled backed.
portal 5.3.1
This is a known issue and will be fixed in portal 5.3.2
Workaround :
Make the update directly in the portal database .
mysql> update TENANT_INFO set portal_name="NEW Portal name " where portal_name="Current portal name ";