While I haven't changed a battery on a TX-7 it is most likely soldered to the board which was typical of most electronics gear at this time. So you would have to unsolder the battery and resolder the new one. I think it was lack of foresight on the part of electronics companies that never envisaged their products still being in use a decade or more later and needing to replace batteries.
Although I don't know of TX-7 schematics, apart from the keyboard related circuitry the DX-7 schematics should be pretty close and Zicweb
has a collection of user manuals and service schematics that may provide some useful info.
[This message has been edited by Nigel (edited 05-31-2002).]