Do you need to need to change the thermal paste?
If you bought that T440s new back in 2013-2014, and haven't done any sort of maintenance, then yes you need to..
If you bought it used more than 2 years ago, and haven't done any sort of maintenance, then yes it's good idea to do so.
If you just bought it recently, monitor the temperatures with software like Open Hardware Monitor
If you need to replace the paste, you gotta learn how to open up the laptop. Grab a copy of the T440s Hardware Maintenance Manual (HMM). Google is your friend.
The HMM contains the necessary disassembly instructions. Give it a good read. to get a good grasp at the steps that need to be done.
Acquire the necessary tools and consumables:
Philips #1, #0 and #00 screwdrivers
Thermal paste (Thermal Grizzly Kryonaut is currently the best)
lint-free cloth (some use coffee filter paper)
isopropyl alcohol (70% is usable, >90% is better) or thermal paste cleaning compound, like Arctic Silver Articlean
small, fine brush (personally I use an old toothbrush)
compressed air can or hand-operated air blower (usually sold for cleaning camera lenses)
(optional) angled needlepoint tweezers or plastic spudger
Since it's your first time, set aside an entire afternoon. This way you won't be pressured into hastening your pace.
Remember: working too fast or getting too excited usually leads to mistakes.
So work at a slow deliberate pace.