You can grant as below instead of altering each table's owner.
I faced below error while updating similar table,
PG::InsufficientPrivilege: ERROR: must be owner of relation <my table name>
Suppose a table was created by a DB user named "createU" and you are trying to perform other operation using another DB user named "updateU" ...
Stop the primary service
systemctl stop postgresql ( This command might be different on your OS )
login to the secondary and promote it to primary ( -D is for the data directory )
pg_ctl promote -D /opt/pgsql/data
Now at this point you need to change the settings on the old primary to become the new replica and start the service again