I have deployed a WebApp based on JavaEE on a server and I have a lot of Java Servlets.
Many times, I found an error inside a servlet so I fix it. I know that I can upload the WAR file to the server and deploy it to got the changes, but... I find it too much load just for 1 servlet!
So, how would I proceed to update the server? Is it possible to upload the servlet class file and that's all?
Dropping off the war is the recommended procedure. It's usually possible to do what you're asking but I would recommend against it.
Usually the app container will explode the war after you drop it off into some folder. You could overwrite the class there directly. You would then have to restart the app container anyway to load the class. I did this a couple of times for some legacy apps I did not write and had difficulties building, so I didn't have a war, if I had a war I would not do this.