The only common denominator to all those devices is your router.
To access its settings you need to know the address. Press your Windows key and R together and type
cmd /k ipconfig /all
into the Run box then press the Enter key.
In the black Command form that shows up will be some details of your network and you need the address next to Default Gateway. It might look similar to 192.168.1.1.
Put that address into the URL line in your browser as though it were an Internet address. You will need to log in using details that, hopefullly, will be printed on the routers case somewhere. The log in is sometimes admin and the password is password but check first.
Once you're in, find the Wireless Security key and change it to a more complicated one, using capital letters and some numbers along with a £, $ or % if it allows those characters. Write it down somewhere - all your wireless device wlil need it to reconnect when you've finished.
While you're in there, change those log in details, again using a very strong password.
That should stop the neighbour who smirks at you in the street from getting back in.