RRDTool | Edit RRD File Export and Import
Here I will discribe how to edit a rrd file by export and import them
Command Line: Export a rrd File into xml
rrdtool dump /usr/local/pnp4nagios/var/perfdata/HOST1/PING.rrd /usr/local/pnp4nagios/var/perfdata/HOST1/PING-EXPORT.xml
Now edit the XML File like your wishes and reimport them. Attention! You will override the existing RRD File so you could lose data which has been collected between your export and import time!
rrdtool restore -f /usr/local/pnp4nagios/var/perfdata/HOST1/PING-EXPORT.xml /usr/local/pnp4nagios/var/perfdata/HOST1/PING.rrd