Go to your Control Panel (exact process will depend upon your Windows version) and select Network and Sharing Center.
This should list all your current network connections.
Select the required network connection (often called Local Area Connection) by clicking on the blue name.
In the window that pops up, choose details and look for the IPv4 details for your IP Address, Subnet Mask and Default Gateway.
To change these settings, go back to the previous window and choose Properties.
Select Internet Protocol Version 4 (TCP/IPv4), then Properties.
In the new window, you can set the IP Address, Subnet Mask and Default Gateway.
Go to your System Preferences > Network
Select your Network Connection.
For wired connections, change the dropdown to Manually and enter the required network settings.
For Wireless connections, select Advanced, then TCP/IP then change the dropdown to Manually and enter the required network settings.