Hi there @iamhimanshusrivastava,

You should be able to install Php 7.3 by running the following commands:

sudo add-apt-repository ppa:ondrej/php
sudo apt-get update
sudo apt install php7.3

Installing most of the common Php packages can be achieved by using:

sudo apt-get install php7.3-fpm php7.3-cli php7.3-mysql php7.3-gd php7.3-imagick php7.3-recode php7.3-tidy php7.3-xmlrpc

You can then restart Php & Nginx with the following:

sudo systemctl restart nginx.service
sudo systemctl restart php7.3-fpm

Once that is done, nginx should be using php 7.3 - You can verify this by creating a new php file at the root of your website with the following contents:

<?php
phpinfo();

You can then access this file in a browser to confirm that the correct version of php is running.

Hope that helps!
- Matt.

additionally to steps from MattIPv4, to enable apache2 to use php7.3 run these:

sudo a2dismod php7.2

a2enmod proxy_fcgi setenvif

a2enconf php7.3-fpm

If you are using nginx after upgrading php you also need to change the directive in nginx’s default configuration file:

vi /etc/nginx/sites-available/default

Find the line:

fastcgi_pass unix:/run/php/php7.0-fpm.sock;

And change it accordingly (7.3 in this example):

fastcgi_pass unix:/run/php/php7.3-fpm.sock;

Otherwise nginx will still use the old version.