Using iptables -F PREROUNTING you can delete all rules for chain PREROUNTING.
Using iptables -D PREROUTING 1 you can delete a single first rule from chain PREROUTING. So to delete above two rules you will have to use the same command twice.
Use iptables -t nat -D PREROUTING -p tcp --dport 12348 -j DNAT --to-destination 192.168.0.5:12348 and iptables -t nat -D PREROUTING -p tcp --dport 7778 -j DNAT --to-destination 192.168.0.5:7778 to delete these two specific rules.
Just replacing -I with -D.