Hi Ichiro san,
Please see the following KBA Switching a GPIO Pin Connection between Analog and Digital Sources in PSoC® 4 - KBA90272 . Meanwhile first you have to identify the pins which are shared between the resources you want. In your case it is UART and I2C. This can be found using datasheet pinout table. There all pins and their alternate functions are tabulated. HSIOM registers are manipulated to switch between the resources. They are accessed through register write functions. You can check the KBA as reference. If you have doubts regarding the register, you can check the registers TRM for the device.
Thank you for your reply.
I will try programming with reference to KBA 90272 I introduced.