Are you talking about downgrading or upgrading?
For both there are several methods to do it (see the wiki link i posted).
Regarding downgrading, for me personally the simplest way is to download the boot.tar.xz file and extract it to the boot medium.
For upgrading just go into Batocera, press Start, go to Updates & Downloads, set the update type you want (Stable for the current stable which is v36 or BETA for the current beta, which is v37) and then Start Update.
DJ151 when I go to up dates for the pi 3b it shows “ no updates available ”.
Which update type did you set? Stable or BETA?