Before you try Fossapup64 consider this and ask yourself what are your objectives:
Puppys are modular by design. Its components consist of:
(a) initrd(.?g) --the initial RAM disk which is loaded into RAM containing instructions as to which other components to copy or mount in RAM;
(b) the kernel group: vmlinuz, zdrv_XXX.sfs and sometimes fdrv_XXX.sfs. vmlinuz is the 'engine, and drive-train' of a Linux operating system, for and upon which the other components are constructed and by which they are powered. zdrv_xxx.sfs are drivers [of a computer's hardware] which must be compiled against vmlinuz. fdrv_xxx.sfs is the firmware also required to communicate with a computer's hardware, but can be used with any vmlinuz. [Devs publishing Puppys or 'hughe-kernel' packages sometime package both firmware and drivers in the zdrv.sfs].
(c) Puppy_XXX.sfs --this is the 'body' of the system. It includes the file-manager and window-manager; those applications the 'average' User will want to perform tasks such as browsing the web, writing a letter, draw a picture, listen to a music collection; and the infra-structure to 'link' those to the kernel-group.
(d) adrv.sfs and/or ydrv.sfs --optional additional applications the Puppy's developer thought some users might want. If present adrv.sfs and ydrv.sfs will always be copied into RAM on boot-up. Anyone can package together any group of applications as a adrv.sfs or a ydrv.sfs. These are just 'squashed-file-systems' initrd instructs to make use of if present. [And Puppys can make use of other file-systems --packaged as SFSes, or as AppImages-- which need not be loaded on boot-up].
Because the 'kernel-group' is independent of the 'body', it is easy to 'swap' the kernel-group which came in the Fossapup64 ISO for that which came in the Bionicipup64 ISO; thus 'upgrading' Bionicpup64. Or look here for even newer 'kernels' or even older ones if a newer kernel-group does not provide the drivers required to communicate with an older computer's hardware. viewforum.php?f=65.
With two exceptions applications a User employs to 'do stuff' never have to be 'updated/upgraded'. The latest 'fire-fox' will happily run OOTB under Xenialpup which was released 5 years ago. Tahrpup-era (published 7 years ago) LibreOffice will happily run OOTB under Fossapup.
One exception is that the User needs the 'bells and whistles' only available in a newer version of an application. That exception often comes with a price. Newer applications may have as dependencies Qt5, gtk3 and similar 'new-fangled' infra-structures while older application required the older, smaller and less RAM-demanding analogs Qt, gtk2 and similar.
Usually if a Puppy does not include the infra-structure required by an application, that infra-structure can be installed.
The above explains why Bionicpup64's Quickpet does not provide anything newer than 2020. Once Ubuntu 'fixed' any latent bugs in Ubuntu Bionic Beaver and those fixes had been incorporated into Bionicpup, there was no reason to 'update' or 'upgrade' the base system. Specific software upgrades are a choice left to the User's needs and discretion.
The other exception is more limiting. Security demands and graphic possibilities by Google, Mozilla and Websites require that Web-browser be constantly updated to provide or comply with that security or permit the proper rendering of content.
Puppys can not always be retro-fitted with the ‘infra-structure’ required by currently acceptable web-browsers. Without getting into specifics, the infrastructure of 5 year old Xenialpup is just barely acceptable while that of Tahrpup may not be and could not be retro-fitted to comply with Google’s demands. I think it can still run current ‘Mozillas’. [I no longer have my computer setup to boot into Tahrpup so I can’t easily check].