Download the files bootloader.bin, image-x.y.z.bin and pinetime-graphics-x.y.z.bin from the release page:

The bootloader reads a boot logo from the external SPI flash memory. The first step consists of flashing a tool in the MCU that will flash the boot logo into this SPI flash memory. This first step is optional but recommended (the bootloader will display garbage on screen for a few second if you don't do it). Using your SWD tool, flash pinetime-graphics-x.y.z.bin at offset 0x0000. Reset the MCU and wait for a few seconds until the logo is completely drawn on the display.

Then, using your SWD tool, flash these file at the following offsets:

• bootloader.bin : 0x0000
• image-x.y.z.bin : 0x8000

Reset and voilà, you're running InfiniTime on your PineTime!