How to update c++ or g++ to version 14 or above? [closed]
There is no g++15. Ubuntu 20.04 comes with gcc-9 by default (and gcc-10 can be installed with sudo apt install gcc-10). g++ is a part of GCC.
According to Wikipedia,
GCC has had complete support for C++17 language features since version 8
Therefore, gcc-9 certainly supports C++14. There is no C++15.