How I can mutate a struct's field from a method?

You need to use &mut self instead of &self and make the p variable mutable:

struct Point {
    x: i32,
    y: i32,
}

impl Point {
    fn up(&mut self) {
        // ^^^ Here
        self.y += 1;
    }
}

fn main() {
    let mut p = Point { x: 0, y: 0 };
    //  ^^^ And here
    p.up();
}

In Rust, mutability is inherited: the owner of the data decides if the value is mutable or not. References, however, do not imply ownership and hence they can be immutable or mutable themselves. You should read the official book which explains all of these basic concepts.

By using Cell<T> you can emulate field-level mutability:

use std::cell::Cell;

struct Point {
    x: i32,
    y: Cell<i32>,
}

impl Point {
    fn up(&self) {
        self.y.set(self.y.get() + 1);
    }
}

fn main() {
    let p = Point { x: 0, y: Cell::new(0) };
    p.up();
    println!("y: {:?}", p.y);
}

This will print y: Cell { value: 7 } and we've successfully updated y.

Additionally, if you are using nightly channel, you can declare #![feature(cell_update)] on top of your .rs file and use the following syntax inside your up() method:

impl Point {
    fn up(&self) {
        self.y.update(|x| x + 1);
    }
}

Note: This feature above is a nightly-only experimental API.

From The Rust Programming Language at Rust 1.7.