Many time, you will need to load another operating system than the one you are currently using, just to modify some things on your operating system. The system will then live on your RAM and not on your hard drive.
To do this, you need to prepare a USB key (usually more than 8GB) dedicated for this. You will download the installer of some popular linux distributions that offer this possibility. Here, we will take the example of Ubuntu.
You should download the last version of this OS here.
Follow the instruction here to verify that you downloaded the right software.
You should then flash it to your USB key using the software balena etcher.
After this, keep the device plugged in and restart your computer. At the second you see the BIOS screen, enter del and all the F keys to enter into the BIOS configuration. Here you can select to boot with higher priority on the USB key, and select it to boot.
Then, click on “Try Ubuntu”, not “install”. You will then see a version of ubuntu running “live”. You can then open a console and access your hard drive.