During the solution deployment in SharePoint, if you are trying to upgrade a solution and the deployment fails in any Server in the farm, there can be a possibility that it is trying to overwrite on an existing DLL and the same DLL was being used or locked by an existing process, which was running in the Server. Hence, we need to find out which process is locking the DLL.
Solution
Open the command prompt and run the command given below.
tasklist /m <thelocked.dll>
This will show all the processes that are currently using that DLL and you need to restart the Services for those processes.
If the solution given above does not help, you may need to reboot the Server.
I hope this it helps.