Pin functionality multiplexing in NXP p1010 processor
That is a pretty broad question.

So, most microcontrollers have some way, typically some control registers, that you can use in your firmware to change the functionality of a pin.

Now, if you need to "multiplex" that pin to different signals/elements of your circuitry, you'll need some kind of multiplexing.

In case of digital signals, that might simply be some transistors that you address with another pin; in case of analog signals, or fast digital signals, this might be a bit more complicated and need specific analog switches – whether those are discrete-transistor-based, based e.g. on analog switch ICs (e.g. from TI, or ST. Basically, go to any semiconductor manufacturer's website, and search for "analog switch") , or even relays, completely and utterly depends on your application. Furthermore, you'll still need some output to program these multiplexing devices – and that will require more output pins, and it seems you're already in a situation where these are sparse.

Generally, I kind of feel like you're doing something wrong: If you already use a relatively large microcontroller, just use one with even more pins, and simply have one pin for each part of your schematic. If you can't do that, find the digital outputs or inputs that you don't need to switch/read fast, and attach them to some sort of IO extender – be it a simple shift register, or some kind of I²C-connected IO-extender IC, or a CPLD, or something connected to some high-speed bus, or any other way of using few pins to provide many IO pins.