Alright, let's do this. Fast.
Disclaimer: this only works when the /home directory is on the same partition, which is the default option if you don't specify.

Step 1: Boot up a live environment.

For the sake of simplicity, I'll be using the Endeavour OS Galileo installation media, but any linux distro should work

When you're in, open the terminal:

Step 2: Mounting the linux distro

Type in lsblk. This will show all mounted drives.

Locate the drive and partition where your installation is.
It's usually the partition with the most space. The space is on the size row (duh)
If you have multiple drives with the same size and want more info about the volumes, type in fdisk -l.

In my case it's /dev/sda1.

So let's mount the partition to the /mnt directory with sudo mount /dev/sda1 /mnt.

Step 3: Chrootin'

Chroot is a linux tool which basically changes the root directory to whatever directory you specify. This will be used to run the passwd command inside your installation's context.

Arch Linux has it's own chroot command which does some magic in the background to make it useable on this distro's environments.

sudo arch-chroot /mnt

should chroot into your installation and after a few seconds a shell will show up!

And now one last command, the one that actually changes the password:

passwd yourusername

and boom! that's it! impressive, right? exit off the console and then reboot.

The end

That was quick.