Hi,
Here’s a link to the [http://tim.id.au/laptops/lenovo/thinkpad%20t400s%20t410s%20t410si.pdf|service manual] for the laptop.
Scroll to p.103 to view the necessary pre-requisite steps and then the procedure to remove/replace the backup battery.
The part number for the battery is ''45N5864'' (see p.174 Item #19)
If you search online using the part number ''only'' in the search box of your browser, you’ll get results for suppliers of the part.