B Physics at the HL-LHC with the upgraded CMS detector

The high luminosity LHC (HL-LHC) run, which is due to start in 2026, is expected to deliver an integrated luminosity of approximately 3000 fb$^-1$ at a pp center of mass value of 14 TeV. Significant upgrades of the CMS detector are foreseen to withstand the highly-demanding operating conditions and to fully exploit the delivered luminosity. More precise investigations of rare decays in the flavour sector will be possible thanks to the large collected data sample. The perspectives for the measurements of the $B^0_s \to \mu^+\mu^-$ and $B^0 \to \mu^+\mu^-$ branching fractions are reported, together with the projections for the exclusion limit on the $\tau \to 3\mu$ branching fraction.