Partial file encryption c#

Question

Good day everyone,
I'm doing some job with file encryption and want to do partial file encryption.
Let's say i have text file with data:
123456789999999888844565452 and want to encrypt only part of file
xxxxxxxxxx999999888844565452 - where x encrypted part.

How I can do this ?
I've tried to do this but everything failed because when decrypting algorithm throws "invalid password" error.