How to remove nginx and re activate apache?

First, you need to stop nginx so it releases port 80 so that apache2 can listen to it later on.

sudo service nginx stop

Next, if nginx was installed with apt-get, it can be removed by running one of the following two commands (I explained the difference in the next paragraph).

sudo apt-get remove nginx

Or

sudo apt-get purge nginx

Both of them remove all package files, while the second also removes the configuration files that package had installed.
If you intend to use nginx later on with the configuration you did, use remove. Else, I would suggest using purge.

After removing nginx, you can restart apache to make sure it is listening to port 80.

sudo apache2ctl restart

If you had removed apache before installing nginx, you can re-install it with

sudo apt-get install apache2

if after that the Welcome to nginx! page appears you can try unistall other version on nginx existing on your server with

sudo apt-get remove nginx-*

and

sudo apt-get purge nginx-*

Remove all nginx packages

sudo apt-get autoremove nginx
sudo apt-get purge nginx

This works for me

sudo service apache2 restart