This short tutorial is in regards to parameter window https://www.amibroker.com/guide/w_param.html
Sometimes you just want to reset a single parameter of "Parameters" tab back to its default value but not all parameters. You can do this very easily by opening the parameter window (either via CTRL+R or by right clicking on chart and choosing "Parameters" in context menu or if being in Analysis mode you click this button 
there).
Now that you have opened parameters window the next thing to do in order to reset just one single parameter is going to your parameter's description bar on the left side of that window and executing a double-click via right mouse button.
As aside a reset applied via upper mentioned technique will not work if a parameter is defined via ParamToggle. I guess it is by design since it is just an "ON/OFF" setting.