I accidently erased parts of the internal memory of my Garmin 830

I accidently erased parts of the internal memory of my Garmin 830. By Garmin Express its not possible to reinstall any files, not enough memory. But there are no big files anymore in the memory. Is it possible to format the memory and to reinstall all files? Where can I get these files. 
Garmin wants, that I change it for a new one, which I have to pay. 
Sorry about my English. Greetings Uwe from cologne.