Not very easy.
Contacts are stored in an sqlite3 database in ‘/home/defaultuser/.local/share/system/privileged/Contacts/qtcontacts-sqlite/contacts.db’
You must have privileged access to alter it (‘devel-su -p’).
If your contacts are saved and you can resynchronize them, the easiest way is to delete contacts.db and resynchronize your contacts.
For Calendar, the sqlite3 dabase is in ‘/home/defaultuser/.local/share/system/privileged/Calendar/mkcal/db’.
Idem if you are sure you can resynchronize your calendar, the easiest way is to delete file ‘db’ and resynchronize.