It's not, really.
C++ has a standard library that isn't in C. You can change stuff over, but it's effort.
Also, C++ introduces classes. C does not have classes. You could try fudging them with structs and function pointers or something, but again, totally don't recommend that.
If you know C++, you can pick up on C in about a weekend, since you've accidentally learned most of it via C++.
and if you know C, you can pick up on C++ in about a week.