Hi and welcome to the forum.
I assume you are talking about metrics? They are stored in the site directory in these files:
~/var/check_mk/rrd/HOSTNAME/SERVICE.rrd
~/var/check_mk/rrd/HOSTNAME/SERVICE.info
The *.info file contains some description and the *.rrd file the actual data.
Usually the files contain the data for all the graphs of a given service. So in your case both the “old” data and the “new” data is stored in these files.
By far the easiest way to get rid of the old data is to stop the site and delete these two files completely, i.e.:
omd stop
rm ~/var/check_mk/rrd/HOSTNAME/SERVICE.*
omd start
The files will be re-created automatically.
However, this will also delete the “new” data collected so far.
If you prefer to keep the “new” data, then it might get a bit complicated but here’s a description: [Check_mk (deutsch)] Performance Data gezielt löschen (it’s in German, though).