It doesn't take much time to install flashplugin. To check whether update is still working (downloading/installing) check the output of this command:

ps aux | grep http

This will result in 2 or 3 lines with the second column of each line being the pid of the process if the process is doing its job. If the result is just 1 line, say the last phrase being grep http or something, the updates are not being downloaded/installed. Its safe to kill the update manager

To do this:

run: ps aux | grep update and note the pid of the update-manager being the second column after your user name in the 2/3 line list of output. Let the pid of update-manager be pid

Kill this process by running:

kill -9 pid

If operation is not permitted, try with sudo, but take care that you don't kill any other process. Note the pid of update-manager carefully.