Are you looking for Lower Franconia in General or for the city of Würzburg in particular?
For the city of Würzburg:
For records between 1811 and 1875 (marriages until 1919), contact the Würzburg State Archive: poststellestawu.bayern.de
For records after 1875 and older than 110 years (birth), 80 years (marriage) and 30 years (death), contact the Würzburg City Archives stadtarchiv@...
For later records, you must be a direct descendant of the person you are looking for. Contact the Würzburg Civil Record Office at standesamt@... or request a document online at https://www.wuerzburg.de/buerger/standesamt/urkundenbestellung-online/index.html
The birth records between 1876 and 1945 were destroyed by fire during the war, except for the boroughs that were merged later.
The following web page (in German) gives general instructions for genealogical research for the city of Würzburg:
Andrea Schwab, Montreal, Canada