log in as root and use either mii-tool or ethtool
examples are :
Code:
# mii-tool -F 100baseTx-HD
# mii-tool -F 10baseT-HD
or
Code:
# ethtool -s eth0 speed 100 duplex full
# ethtool -s eth0 speed 10 duplex half
read the man pages or do searching on these tools to achieve exactly what you want.