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

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 presumably have /usr/local/bin ahead of /usr/bin in your PATH, and since there’s a php binary in /usr/local/bin, your shell uses that. The php alternative configured by update-alternatives ends up having no effect — at least, on anything based on that PATH.

To fix this, in your case, I recommend removing the PHP installation in /usr/local. A simpler approach would be to re-order your PATH, but that might affect other binaries you rely on in /usr/local/bin.

EDIT:

As pointed out by ᴍᴇʜᴏᴠ, Debian does support update-alternatives for the socket, but it does it a little differently, and there are two requirements:

• You must be running a PHP-FPM package from Feb 1 2020 or later (which hopefully you are)
• You must be running systemd versus sysvinit

The generic socket, located at /run/php/php-fpm.sock, is created by the systemd unit file by calling /usr/lib/php/php-fpm-socket-helper via the ExecStartPost and ExecStopPost directives. If you wish to use this socket, you still need to ensure your web server config is pointed to it versus the version-specific file.

The original answer is below if you don't meet those requirements.

It appears you are running a Debian/Ubuntu based distribution, which intentionally installs php-fpm in version specific locations (it's the whole reason you can use update-alternatives to switch the default version on the fly). However, there is no alternatives config for the socket files of php-fpm (your call to update-alternatives is only changing the CLI PHP version at /usr/bin/php).

The socket file to use is defined in /etc/php/8.3/fpm/pool.d/www.conf.

; The address on which to accept FastCGI requests.
; Valid syntaxes are:
;   'ip.add.re.ss:port'    - to listen on a TCP socket to a specific IPv4 address on
;                            a specific port;
;   '[ip:6:addr:ess]:port' - to listen on a TCP socket to a specific IPv6 address on
;                            a specific port;
;   'port'                 - to listen on a TCP socket to all addresses
;                            (IPv6 and IPv4-mapped) on a specific port;
;   '/path/to/unix/socket' - to listen on a unix socket.
; Note: This value is mandatory.
listen = /run/php/php8.3-fpm.sock

If you really want to have a version agnostic socket file, you can change that setting to just be listen = /run/php/php-fpm.sock and update your nginx config accordingly. You'll need to do this for every version of PHP you have installed, and it also means you cannot have two different php-fpm versions running simultaneously.

You might be able to play around with creating your own update-alternative config for /run/php/php-fpm.sock that allows you to switch between versions, but I'm not sure how well that will work trying to manage files in /run that way.

you have to install alternative path first to display it in alternatives list, see manual page
by using this command,

update-alternatives --install /usr/bin/php php /opt/plesk/php/7.2/bin/php 71

it will install your /opt/plesk/php/7.2/bin/php into alternatives list

I think the best way to do this on Ubuntu is like that:

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

You may change the version according to your needs.