Unable to self-update Composer

As Waqleh said, you have to uninstall PHP Composer and install it again. First, execute:

sudo apt-get remove composer

Then, execute these commands. The checksum here is for Composer 1.10.13, but you'll get the newest Composer (2.0.4 at the moment of editing this answer) when running the first line, so be sure to check in https://getcomposer.org/download/:

php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
php -r "if (hash_file('sha384', 'composer-setup.php') === '8a6138e2a05a8c28539c9f0fb361159823655d7ad2deecb371b04a83966c61223adc522b0189079e3e9e277cd72b8897') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"
php composer-setup.php
php -r "unlink('composer-setup.php');"

Now move file composer.phar to a directory that is in your path (from Installation - Linux / Unix / macOS):

sudo mv composer.phar /usr/local/bin/composer

And execute composer from any directory. That's all!

PS: If you're using PhpStorm (or maybe other IDEs), you'll have to close it and open it again.

Since I posted my answer, I have learnt a new easier way to install Composer programmatically: How do I install Composer programmatically?

Old Answer:

As per @JimL comment, I was able to self update Composer by:

Uninstalling Composer from the package manager (apt).

I installed it according to the official documentation

Now it works as expected.

Install the latest version:

Remove your current Composer version, for example Ubuntu/Debian:

sudo apt-get remove composer

Now, head to https://getcomposer.org/download/ and paste the script in your command line. This ensures that you get the latest version of Composer (as time of writing: v2.0.7).

Like this:

php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
php -r "if (hash_file('sha384', 'composer-setup.php') === '906a84df04cea2aa72f40b5f787e49f22d4c2f19492ac310e8cba5b96ac8b64115ac402c8cd292b8a03482574915d1a8') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"
sudo php composer-setup.php --install-dir=/usr/local/bin --filename=composer
php -r "unlink('composer-setup.php');"

After some time passed, you can update and there isn't any need to use the sudo prefix:

composer self-update

It worked for me (linux, Ubuntu 20.04):

sudo apt-get remove composer
sudo apt-get update
sudo apt-get install curl
sudo curl -s https://getcomposer.org/installer | php
sudo mv composer.phar /usr/local/bin/composer

If you have an old version of Composer you need to follow these commands:

composer -V
sudo apt remove composer
cd /tmp
wget http://getcomposer.org/download/1.10.5/composer.phar

php composer.phar -V
sudo mv composer.phar /usr/bin/composer
sudo chmod 750 /usr/bin/composer
composer -V

You can specify the installation directory and filename while setting up PHP Composer - php composer-setup.php like so:

sudo php composer-setup.php --install-dir=/usr/bin --filename=composer

Already there are many answers, Would like to share my experience with this.

I am using Ubuntu 20.04, and My previous Composer version was Composer 2.0.11 and my Project requirement was Composer 2.1.15.

I didn't had to remove anything, I simply used the following command and it worked fine for me :)

https://i.stack.imgur.com/nDMnQ.png

I've installed Homebrew and it saves me a lot.

Install brew and then brew install composer to install Composer.

composer 2 on debian :

cd /usr/src

sudo apt-get install curl php7.2-cli

curl -sS https://getcomposer.org/installer | sudo php -- --install-dir=/usr/local/bin --filename=composer

chmod 777 composer

nano ~/.bashrc

add : PATH=$PATH/usr/local/bin/

source ~/.bashrc

verify :composer -v

Install the latest Composer by the following steps:

Uninstalling Composer

sudo apt-get remove composer

Run following commands

php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
php -r "if (hash_file('sha384', 'composer-setup.php') === '8a6138e2a05a8c28539c9f0fb361159823655d7ad2deecb371b04a83966c61223adc522b0189079e3e9e277cd72b8897') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"
php composer-setup.php

Install Composer in the /usr/bin directory to run Composer from anywhere

sudo php composer-setup.php --install-dir=/usr/bin --filename=composer

Remove the installer

php -r "unlink('composer-setup.php');"

To check or self update

composer self-update