Hi @vion ,
Here’s the [https://downloads.dell.com/manuals/all-products/esuprt_laptop/esuprt_latitude_laptop/latitude-e6420_owner%27s%20manual_en-us.pdf|owner manual] for the laptop.
Go to p.99 to view the necessary pre-requisite steps and then the procedure to remove/replace the [https://www.amazon.com/Dell-Latitude-E6420-Power-Input/dp/B009K56XZM|DC-In cable - supplier example only]. The supplier example was only to show what the cable might cost. There are other suppliers online. Just search for ''Dell E6420 DC-In cable'' to get results for suppliers that suit you.