Step 1: Put device into flash mode: Reflashing PureOS - Purism user documentation Also, read through the reflashing instructions as they do a better job of explaining the process. Ultimately, you are simply following those instruction and, instead of installing PureOS, you will be running the script to install PostmarketOS.
Step 2: Download your chosen version of PostmarketOS here: v25.12/purism-librem5 - postmarketOS // Official Images
Step 3: Download librem5-flash-image using this link as provided in the above Reflashing instructions:
Step 4: In a terminal, open it in your downloaded Librem5-flash-image folder. E.g.
cd ~/Librem5-flash-image
Step 5: Place your downloaded PostmarketOS image into the Librem5-flash-image folder. Change the name to something short like: postos.img. (This is not required but I find it much easier).
Step 6: run the following script:
librem5-flash-image postos.img
Alternatively, Loki recommends this command in the terminal:
librem5-flash-image --dir . --skip-flash --skip-cleanup --image 20250919-0754-postmarketOS-v25.06-plasma-mobile-5-purism-librem5.img
However, I did not find it necessary to issue all of those commands. Read Loki’s posts in that forum post for more information.