Left click on the file of the expert advisor or indicator and select copy to copy it.
Open your meta trader terminal and click on File at the top left corner of the terminal. A drop down menu will appear. In the drop down menu, scroll to “Open data folder” and click on it.
You should now see the contents of the data folder. For mt4 double click on the folder named mql4 to open it. On mt5 you should double click on the folder named mql5.
Now you should see a couple of folders with one named experts while another named indicators. If your file is an expert advisor, you should open the experts folder by double clicking on it then you paste the file you copied in there. For indicators, do so in the indicators folder.
Now all you need to do is close the meta trader terminal and open it again. You should then find the installed expert advisor in the expert advisors list or indicators list for indicators.