I created a new software and deployed it to the testing group, but the testing failed. so I checked the configuration and changed "install for user" to "install for system"

Will it automatically take effect, or I need to click something

To my knowledge, it should do it automatically but I prefer always killing the old deployment, and also the old revisions of the application before re-deploying it.

It will take effect after the client received policies (for clients that never ran the installation). Retry is different - based on the return code, but it might work after the app deployment eval cycle ran. 

If you are changing anything in the tabs then it should be fine, but if you change anything in the content, ie delete a file or change part of a batch then you would need to update the distribution points.