To manually change your IP in Windows 7, follow these instructions.
Go to the Control Panel, then Network and Internet, Network and Sharing Center. Then choose either:
Change adapter settings in the left menu, then right click your network adapter and select properties.
Local Area Connection on the right hand side and click Properties.
Highlight IPv4 in the window you now see then click Properties underneath it. This will present you with the IP address settings screen. Most computers will have this set to automatic as DHCP is enabled by default.
All you need to do here is to change it to “Use the following IP address" and fill in the address, subnet mask and default gateway. You can then choose whether to manually assign a DNS server or not.
Once you close those windows, the system should manually assign the IP address. You might lose your network connection for a couple of seconds, then it should come straight back. If it doesn’t then check the IP address you chose to double check it’s in the range.
As I mentioned, if you’re behind a router, you will have a pool of IP addresses to choose from. Make sure you either change the pool to one that includes the IP you want, or to use an IP address in that pool. If you do neither, you won’t get a network connection.