Pwd mkii updates

Hi there
I have a pwd mkii ( no bridge ) firmware 4.2.1 .
I have issues with clicks and pops between tracks , not sure if this is my streamer ( hifi rose rs150b ) or the dac ( pwd mkii )
Is this the latest firmware ( 4.2.1 ) or is there a newer version.
If so can you advise which version? And best way to install ? Is this best done via sd card or usb dongle,
Cheers Pete

This may be of help