you are under Windows?
The data are under ..\ProgramData\opencpn\
Tracks and routes are in the navobj.xml file.
Before muddling with this data make a safety
If you copy the recorded file from the disk into your actual system new tracks and routes will be added.
There is no automatic way to merge two navobj files:
if you have already new data in the new install then you will need to copy those data into the big old navobj file manually.
Study the structure (you did make back-up copies, didn't you?) and if you feel fit to do, merge the two files using a good editor like notepad++