it's a complicated multi-step process:
- first you'd have to find a valid replacement pin, i.e.: a pin where you can have PRU 1's R30 pins from this list. There seem to be some available on P8.20/P8.21, but I am not sure how they interact with the working of the eMMC (the SRM linked above may be of use). Alternatively, P8.27, P8.28 and P8.29 are good to use, but they are currently Bela digital I/O, so you would have to choose for one functionality or the other.
- then you have to set the appropriate pinmux setting on those pins, this is done modifying the device-tree overlay for the Bela cape on your board
- last, you'd have to modify the relevant PRU code (these lines).
Before embarking in this journey you should probably stop for a second and check whether you actually need a pull down on the 4051 pins. They will be actively driven high or low when the code runs, so there should not be a need for a pull-up/down afaict (in fact there already are pull-up/downs on all of those pins internally, as shown by the "Processor Boot Configuration Design" image of the SRM linked above.