This will upgrade your php version to php7.4-fpm when using Nginx web server.

Check your PHP version:

$ php -v

1. Install PHP 7.4

$ sudo apt install php7.4-fpm

2. Install additional modules

$ sudo apt install php7.4-common php7.4-mysql php7.4-xml php7.4-xmlrpc php7.4-curl php7.4-gd php7.4-imagick php7.4-cli php7.4-dev php7.4-imap php7.4-mbstring php7.4-opcache php7.4-soap php7.4-zip php7.4-intl -y

3. Update config files

Open the file located at /etc/php/7.4/fpm/pool.d/www.conf:

$ sudo nano /etc/php/7.4/fpm/pool.d/www.conf

Find the lines below:

;env[HOSTNAME] = $HOSTNAME
;env[PATH] = /usr/local/bin:/usr/bin:/bin
;env[TMP] = /tmp
;env[TMPDIR] = /tmp
;env[TEMP] = /tmp

and delete the semi-colons so that it looks like this:

env[HOSTNAME] = $HOSTNAME
env[PATH] = /usr/local/bin:/usr/bin:/bin
env[TMP] = /tmp
env[TMPDIR] = /tmp
env[TEMP] = /tmp

Ctrl+x to exit, then ‘Y' to save and exit.

Then, open the file located at /etc/php/7.4/fpm/php.ini:

$ sudo nano /etc/php/7.4/fpm/php.ini

find (Ctrl+w) and change the following to your desired settings:

max_execution_time = 300
memory_limit = 512M
post_max_size = 100M
upload_max_filesize = 100M

and find the line:

;cgi.fix_pathinfo=1

and delete the semicolon and set to 0:

cgi.fix_pathinfo=0

Ctrl+x to exit, then ‘Y' to save and exit.

3. Web server configuration

Make sure that your web server correctly uses the PHP 7.4 sockets/modules. Edit your nginx config to change the socket paths (substituting ‘nginx_vhost' with your correct filename, i.e. ‘default'):

$ sudo nano /etc/nginx/sites-available/nginx_vhost

For example, change the lines below from php7.3 to php7.4:

...
fastcgi_pass unix:/run/php/php7.4-fpm.sock;
...

Ctrl+x, then ‘y' to save and exit. Restart PHP and Nginx:

$ sudo systemctl restart php7.4-fpm
$ sudo service nginx restart

4. Change the default PHP version

If you have multiple PHP versions installed on your Ubuntu server, you can set PHP 7.2 as the default:

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

REFERENCES

https://kenfavors.com/code/how-to-upgrade-to-php7-2-in-ubuntu/
https://www.cloudbooklet.com/install-php-7-4-on-ubuntu/