I am clueless as what to do.
Perhaps search for this error message that has been posted more than once.
Or, actually read the messages.
error: conflicting return type specified for 'virtual void Sha1Class::write(uint8_t)'
It doesn't seem too hard to understand this part. You should be able to determine that the return type for this function is void. It conflicts with something that says the return type should be something else. What does it conflict with, and what should the return type be? Well, that is answered in the next message:
error: overriding 'virtual size_t Print::write(uint8_t)'
The Sha1Class class derives from Print, which says that the write() method returns a size_t value. So, you need to edit the Sha1Class header file, and change the return type of the write() method to size_t, instead of void.
Then, you need to edit the Sha1Class source file, and make the same change, AND make the class actually return a value (typically 1).