The easiest method is through network manager:
1- From the top of the screen select the network icon, next to the clock and volume, then click Edit Connections.
2- From the window that opens, go to Wired tab, select your connection (there should be only one connection, if you didn't touch anything). Then click Edit.
3- From the IPv4 Settings tab change Method from Automatic (DHCP) to Manual.
4- Under Addresses feild, click on Add.
5- Enter your desired IP address and subnet mask and click Save, you can also enter an optional DNS server here.
Hope this will help