Which release of PureOS are you using? 5.10 makes it sound like you are still on Amber – but lsb_release -a on the terminal should tell you whether you are on Amber and Byzantium. On Byzantium, 5.15 is available (it should even be default by now).
If you want to stick with Amber (why?), you can find out which kernel releases are available by running apt search linux-image librem5 which should find all installable kernels for the Librem 5.