Upgrade to PHP 8.1

sudo apt install software-properties-common
sudo apt update

Add the Repository

sudo add-apt-repository ppa:ondrej/php

Next, update the system repositories to start using the PPA

$ sudo apt update

$ sudo apt install php8.1 libapache2-mod-php8.1

$ sudo systemctl restart apache2

sudo nano /etc/php/8.1/apache2/php.ini

upload_max_filesize = 32M 
post_max_size = 48M 

Replace the current enabled PHP version with your version.

sudo a2dismod php7.4
sudo a2enmod php8.1
sudo systemctl restart apache2