Use update-alternatives to set the default php version:

sudo update-alternatives --set php /usr/bin/php7.4

If you get an error no alternatives for php, refer to my answer on U&L to add php to update-alternatives ( replace python by php).

Interactive switching mode

sudo update-alternatives --config php
sudo update-alternatives --config phar
sudo update-alternatives --config phar.phar

Manual Switching

From PHP 5.6 => PHP 7.1

Default PHP 5.6 is set on your system and you need to switch to PHP 7.1.

Apache:

$ sudo a2dismod php5.6
$ sudo a2enmod php7.1
$ sudo service apache2 restart

Command Line:

$ sudo update-alternatives --set php /usr/bin/php7.1
$ sudo update-alternatives --set phar /usr/bin/phar7.1
$ sudo update-alternatives --set phar.phar /usr/bin/phar.phar7.1

From PHP 7.1 => PHP 5.6

Default PHP 7.1 is set on your system and you need to switch to PHP 5.6.

Apache:

$ sudo a2dismod php7.1
$ sudo a2enmod php5.6
$ sudo service apache2 restart

Command Line:

$ sudo update-alternatives --set php /usr/bin/php5.6

Source

Do you have PHP5 installed on the system?

You can list all loaded Apache modules with apache2ctl -M:

$ sudo apache2ctl -M

Loaded Modules:
 core_module (static)
 so_module (static)
 watchdog_module (static)
 http_module (static)
 log_config_module (static)
 logio_module (static)
 version_module (static)
 unixd_module (static)
 access_compat_module (shared)
 alias_module (shared)
 auth_basic_module (shared)
 authn_core_module (shared)
 authn_file_module (shared)
 authz_core_module (shared)
 authz_host_module (shared)
 authz_user_module (shared)
 autoindex_module (shared)
 deflate_module (shared)
 dir_module (shared)
 env_module (shared)
 filter_module (shared)
 headers_module (shared)
 mime_module (shared)
 mpm_prefork_module (shared)
 negotiation_module (shared)
 php7_module (shared)
 reqtimeout_module (shared)
 rewrite_module (shared)
 setenvif_module (shared)
 status_module (shared)

If you have multiple php versions, use a2dismod to disable the ones you do not need. From there, you can use update-alternatives to change the default version of PHP:

sudo update-alternatives --set php /usr/bin/php7.4

Note: Be sure to change php7.4 to the version you wish to use.

Once this is done, restart the web server:

sudo service apache2 restart

This should give you what you need.

You can do this with update-alternatives. If you would like to do this interactively, you can do this:

sudo update-alternatives --config php

If you like to specifically choose the PHP version (via an alias or whatnot), you can do this:

sudo update-alternatives --set php /usr/bin/php7.4

Of course, be sure to change php7.4 to the actual version you want to switch between.

You should look to switch PHP version instead of downgrading it. You can have multiple PHP versions running on your server at the same time. You can even have different PHP versions for each of your domains, directories.

I'm adapting these instructions from How to use Multiple PHP Versions with Virtualmin and Nginx.

Add ondrej/php PPA

add-apt-repository ppa:ondrej/php

update apt cache

apt update

install php 7.4 and modules

apt install php7.4-memcache php7.4-imagick php7.4-redis php7.4-bcmath php7.4-intl php7.4-mcrypt php7.4-cgi php7.4-fpm php7.4-mysql php7.4-curl php7.4-gd php7.4-imap php7.4-tidy php7.4-xmlrpc php7.4-xml php7.4-xsl php7.4-mbstring php7.4-zip php7.4-cli php7.4-soap php7.4-gmp php7.4-sqlite3

Now open nginx config file for your domain

nano /etc/nginx/sites-available/domain.com.conf

Update php socket

location ~ \.php$ {
    include snippets/fastcgi-php.conf;
    fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
    fastcgi_read_timeout 600;
 }

Restart nginx

service nginx restart

Try it.