Way easy. Make a mod folder with any name. Make an assets folder in it. Make an XML folder in it. Make a GameInfos folder in that. Inside that copy the GameSpeedInfos file from the standard XML. Go into that with notepad or wordpad and change this (run a search to find the text segment you want rather than scrolling through it all)
Code:
<GameSpeedInfo>
<Type>GAMESPEED_NORMAL</Type>
<Description>TXT_KEY_GAMESPEED_NORMAL</Description>
<Help>TXT_KEY_GAMESPEED_NORMAL_HELP</Help>
<iGrowthPercent>100</iGrowthPercent>
<iTrainPercent>100</iTrainPercent>
<iConstructPercent>100</iConstructPercent>
<iCreatePercent>100</iCreatePercent>
<iResearchPercent>100</iResearchPercent>
<iBuildPercent>100</iBuildPercent>
<iImprovementPercent>100</iImprovementPercent>
<iGreatPeoplePercent>100</iGreatPeoplePercent>
<iCulturePercent>100</iCulturePercent>
<iAnarchyPercent>100</iAnarchyPercent>
<iBarbPercent>100</iBarbPercent>
<iFeatureProductionPercent>100</iFeatureProductionPercent>
<iUnitDiscoverPercent>100</iUnitDiscoverPercent>
<iUnitHurryPercent>100</iUnitHurryPercent>
<iUnitTradePercent>100</iUnitTradePercent>
<iUnitGreatWorkPercent>100</iUnitGreatWorkPercent>
<iGoldenAgePercent>100</iGoldenAgePercent>
<iHurryPercent>100</iHurryPercent>
<iHurryConscriptAngerPercent>100</iHurryConscriptAngerPercent>
<iInflationPercent>30</iInflationPercent>
<iInflationOffset>-90</iInflationOffset>
<iVictoryDelayPercent>100</iVictoryDelayPercent>
<GameTurnInfos>
<GameTurnInfo>
[B]<iMonthIncrement>6</iMonthIncrement>[/B]
<iTurnsPerIncrement>75</iTurnsPerIncrement>...
wherever month increment appears to whatever you want it to be. Save that and put it in the Mods directory and start it as a mod (from Advanced on the main screen once you start civ up)