Introduction
How to replace the battery of a Lenovo ThinkPad T420 laptop. The images and information in this guide were taken from this computer's service manual.
Parts
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            Unlock the battery latch and hold it it the unlocked position. 
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            Remove the battery pack in the direction shown by arrow. 
 
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                            Conclusion
                          
                          To replace the battery & reassemble your device, follow these instructions in reverse order.