I have no personal knowledge of the this battery replacement, however my best guess would be a CR2032, or CR2025 type. The coin/button cells of this type seem to be 3V lithium. If you cannot lift the outer tang assembly vs. the inner recessed oval then it is one piece and tack welded to the battery “-“ terminal. The other side of the battery that you cannot see is held to the spring contact on the board by the spring tension of the neg assembly tang. ?You can peel the negative tang off the battery, but that generally eliminates the spring tension of the negative terminal. ?You would have to recreate that tension and install a new battery.?

This vendor has quite a few button batteries of all sizes and several with pre-mounted tangs that have been tack welded on.