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

Install commonly used php-extensions

sudo apt install php8.1-common php8.1-mysql php8.1-xml php8.1-xmlrpc php8.1-curl php8.1-gd php8.1-imagick php8.1-cli php8.1-dev 
php8.1-imap php8.1-mbstring php8.1-opcache php8.1-soap php8.1-zip php8.1-redis php8.1-intl -y