Generally, you cant and you don't have to. Both Android and iOS are Linux/Unix/FreeBSD-based operating systems. As such, they are already well optimized for broadband connections, so there is little need for tweaking the settings for most users. The OSes are also completely locked, so that they can't be modified by the user directly.
That said, there may be some workarounds - if you can access the OS via a shell, you may be able to navigate to/change the TCP/IP settings. Note this may require rooting the device to be persistent across reboots.
You can connect to Android devices using
ADB (Android Debug Bridge) command-line shell utility included with Google's Android SDK. This may allow you to modify the TCP/IP settings using the Linux tweaking guide below.
We have a list of Linux tweaks here:
Linux broadband tweaks.
