The battery is on the front panel PC board -- Significant disassembly -- get the Service Manual for diagrams!
Battery is either on top of the large IC (IC201), shown in TM-241A Service Manual,
or on opposite side of PC board.
You MUST be proficient in proper soldering skills and static protection (ESD) for removal and replacement -- this is why instructions were never provided for consumer/non-technical user replacement.
IF you have never performed work like this -- GET HELP. You see the "bricked units" on eBay, often as parts radios -- missing screws and parts.
Some destroy the microprocessor underneath and others the PC board.
You would think some of these guys were using blow torches -- no skill/talent and their big mistake was opening the radio.
The Lithium battery for TM-241A is Kenwood part number: W09059905 Alternative to Kenwood part number: W09032605