So if i reconfigure,should backup config before?and what configure i do
First off, there is no need to run ./configure as sudo. It is a part of
the install process (usually ./configure, make, sudo make install).
Secondly, ./configure has nothing to do with config files, it is just a
step in the gnu toolchain build process.
"make install" does not overwrite existing configs. However, it is
always prudent to make a backup of your nginx.conf