WIFI doesn't work.

[OmegaPi]

Good Morning,
I just had a raspberry pi 3, for my birthday and so happy. There is something quite interesting, my wifi is really bad, sometime it works and sometimes not, i'm using either wifi or an ethernet adapter because the ethernet cable doesn't work, it shows that the link is up but without connection. However when I try to go to the wireless and wire settings I have this message : Error saving /etc/dhcpcd.conf
The dhcpcd configuration file is not writeable.
I tried to check on this forum and tried somethings : purge the dhcpcd (but didn't purge because I didn't have it) Then I installed it, but after nothing worked, so I purged it and I'm back to 0 !
Any command you wish me to give you ?
Thank you for your help !
Nathan

[SurferTim]

$ sudo nano /etc/dhcpcd.conf
Make your changes, then save them. Works for me.

edit: Your wifi settings are in /etc/wpa_supplicant/wpa_supplicant.conf

[klricks]

For GUI WIFI setup you need to LEFT click on the network icon. Find your SSID and click on that. Enter your PSK.