SwitchHand allows you to change the Main Hand setting you can find in Options > Skin Customization... > Main Hand between right and left with a hotkey. Making you ambidextrous!
The default hotkey to change your main hand is v it can be changed in the keybinds menu, in the "Inventory" category under "Switch Main Hand".
This mod allows you to do something that usually not possible in vanilla Minecraft. Some public server might not allow it, make sure you are allowed to use it if you play there.