We'll be using port number 3000 as the example

Ubuntu

Option 1:

Step One:
Get the process id

sudo netstat -lpn |grep :3000

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You will get an output similar to this

tcp6  0  0 :::3000    :::*    LISTEN    1234/node

Step Two:
Now that you have the process id - which is 1234 ​in this example
Kill the process using the code below

kill -p 1234  OR  kill -9 1234

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Option 2:

fuser -k 3000/tcp

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Windows

Step One
Open up cmd.exe (we may need to run it as an administrator, but this isn't always necessary). Run the below command:
netstat -ano | findstr :<PORT>

netstat -ano | findstr :3000

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The arrow shows the PID (process identifier). Locate the PID of the process that's using the port you want.

Step Two
taskkill /PID <PID> /F

taskkill /PID 4692 /F

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Step Three
You can check if the operation was successful by re-running the command in "Step 1". You shouldn't see any more search results for that port number.