Serial ports are hardware based, and cannot be remapped. You can however remap software serial ports. Keep in mind not every pin supports softserial either, as an available hardware timer is required. Typically you can use:
- UART TX & RX pins
- PPM Input pins
- LED pins
- Motor PWM output pins
If you have one of those free, proceed.
Enable softserial from the Betaflight Configurator "Configuration" tab, then click "Save and Reboot".
Go to the "CLI" tab and type "resource SERIAL_TX 11 A09", hit enter, then type "save" and hit enter.
Go to "Ports" tab and assign the feature you want to "SOFTSERIAL1".
Keep in mind that software serial is limited in terms of how fast it communicate information and the load it puts on the CPU. If you have a very intensive low-latency connection, typically try to avoid use softserial. Softserial is best suited to things like VTX control (smart audio and tramp protocol, etc), GPS, and smart-port telemetry, where latency, speed, or loss of signal during flight isn't an issue. While it can be used for other more system critical links like RC control, I would generally recommend against it, though it is theoretically possible.