============================================
追記
yumでのnodejsのバージョンアップにはまった話と解決方法 – Qiita

結論：古いrpmが残っておりそちらが優先されていた

# ll /etc/yum.repos.d/ | grep node
-rw-r--r--  1 root root  472 Jan  5  2018 nodesource-el7.repo
# rm /etc/yum.repos.d/nodesource-el7.repo
rm: remove regular file ‘/etc/yum.repos.d/nodesource-el7.repo’? y
# yum clean all

============================================

入ってないのを確認

[root@10 ~]# yum remove -y nodejs npm
Loaded plugins: fastestmirror
Repository nodesource is listed more than once in the configuration
Repository nodesource-source is listed more than once in the configuration
No Match for argument: nodejs
No Match for argument: npm
No Packages marked for removal

インストール

[root@10 ~]# curl --silent --location https://rpm.nodesource.com/setup_8.x | sudo bash -

## Installing the NodeSource Node.js 8.x repo...


## Inspecting system...

+ rpm -q --whatprovides redhat-release || rpm -q --whatprovides centos-release || rpm -q --whatprovides cloudlinux-release || rpm -q --whatprovides sl-release
+ uname -m

## Confirming "el7-x86_64" is supported...

+ curl -sLf -o /dev/null 'https://rpm.nodesource.com/pub_8.x/el/7/x86_64/nodesource-release-el7-1.noarch.rpm'

## Downloading release setup RPM...

+ mktemp
+ curl -sL -o '/tmp/tmp.KfgLZx2d66' 'https://rpm.nodesource.com/pub_8.x/el/7/x86_64/nodesource-release-el7-1.noarch.rpm'

## Installing release setup RPM...

+ rpm -i --nosignature --force '/tmp/tmp.KfgLZx2d66'

## Cleaning up...

+ rm -f '/tmp/tmp.KfgLZx2d66'

## Checking for existing installations...

+ rpm -qa 'node|npm' | grep -v nodesource

## Run `yum install -y nodejs` (as root) to install Node.js 8.x and npm.
## You may also need development tools to build native addons:
##   `yum install -y gcc-c++ make`

[root@10 ~]# yum install -y nodejs
Loaded plugins: fastestmirror
Repository nodesource is listed more than once in the configuration
Repository nodesource-source is listed more than once in the configuration
Loading mirror speeds from cached hostfile
 * base: ftp.iij.ad.jp
 * extras: ftp.iij.ad.jp
 * updates: ftp.iij.ad.jp
Resolving Dependencies
--> Running transaction check
---> Package nodejs.x86_64 2:6.13.0-1nodesource will be installed
--> Finished Dependency Resolution

Dependencies Resolved

================================================================================
 Package      Arch         Version                       Repository        Size
================================================================================
Installing:
 nodejs       x86_64       2:6.13.0-1nodesource          nodesource        13 M

Transaction Summary
================================================================================
Install  1 Package

Total download size: 13 M
Installed size: 38 M
Downloading packages:
nodejs-6.13.0-1nodesource.x86_64.rpm                       |  13 MB   00:00
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
  Installing : 2:nodejs-6.13.0-1nodesource.x86_64                           1/1
  Verifying  : 2:nodejs-6.13.0-1nodesource.x86_64                           1/1

Installed:
  nodejs.x86_64 2:6.13.0-1nodesource

Complete!

[root@10 ~]# node -v
v6.13.0

えぇ。。。