Your IP address is assigned to you by your ISP. You can't change it within the router itself.
A few servers use "static" IP's, but most are "dynamic" and change every so often.
If you want it to change faster, the easiest thing to do is temporarily disconnect your internet connection (just unplug your modem from the wall for a little while). If there is no activity on your IP, eventually your ISP will assign it to another user. When you reconnect, your ISP will then automatically assign you a new one.
Different ISP's have different time limits before doing this. On some, it takes less than 24 hours. On mine, it takes a few days.
Doing this does not have any effect on your computer or internet connection, at least that I know of.
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How to change ip address? ? 
