Several iPhone users have reported an issue where WiFi connection repeatedly drop, reverting to EDGE connectivity. Like many iPhone troubleshooting issues, there are a number of potential solutions for this problem that sometimes need to be applied in tandem. Run through this list of fixes in order until the issue is resolved:
Modify router settings Try modifying the settings on your wireless router in the following ways: Enable the DHCP server increase the maximum number of users. Change the starting distributed IP address to something with only one digit at the end (e.g. 192.168.1.6). Extend the client lease time.
You can also try modifying the security settings on your router. Switch from WEP to WPA or vice versa and from high to low security or vice versa.
Restart router Simply unplugging your wireless router from power then reconnecting it can allow persistent iPhone WiFi connectivity.
Renew DHCP lease Go to Settings > Wi-Fi, then tap the arrow next to your current WiFi network and tap "Renew Lease."
Reset iPhone Simply powering the iPhone off then turning it back on may resolve this issue. Hold the sleep/wake and home buttons simultaneously until the white Apple logo appears.
Reset network settings Go to Settings > Reset > Reset Network Settings. Note that this will erase any currently stored WiFi passwords.
Restore iPhone Connect your iPhone to your Mac or PC and, in iTunes, click the Restore button under the Summary tab. Restoring the phone will erase contacts, calendars, photos and other data on the phone, but will restore automatically backed-up information including text messages, notes, call history, contact favorites, sound settings, widget settings, etc.