Depending on the amount of available Trade Nodes, it should be possible to reassign a farm to another city by simply left-clicking on the farm and then right-clicking on your desired city.
If you want to connect the farm to a city but there's no available Trade Nodes, select one that's currently in existence and then you should have a menu on the bottom-right of your screen. On that menu you will have the option to destroy the particular Trade Node. Do so and then left-click on the farm and then right-click on the desired city.
This is also possible for Trade Nodes over sea, with the exception being you left-click on one coastal city and right-click on another. :)
Cavalry charges rely on the fact that your horsemen absolutely need to have stamina available and to also have a suitable distance in between them and their target. The distance should be such that your cavalry do not expend all of their stamina before they reach their target. My advice: walk your cavalry everywhere until an enemy is spotted and then, when there's a small but suitable distance available, charge the enemy unit(s). You will know if your cavalry pulled off a charge by a small modifier that appears after a charge has been executed successfully.
Finally, what trouble are you having with unit groupings?