The newer way to make the job is to use ethtool
ethtool -s ifname speed 100 duplex full autoneg off
Obsolete, but sometimes still working is mii-tool
mii-tool -F 100baseTx-FD ifname
Where ifname is the name of your network interface, such as eth0.
In both examples the network interface speed is set to 100 Mbit with full duplex and disabled autonegation to keep this settings.