How would I go about turning this code into so I give the parameters to the method by hand and it returns a 2D char array back?

i would not ask the user the height but import it manually.

First, I recommend that you treat height as a parameter to your function.

BEFORE:

    public static void tehtava7() {
        java.util.Scanner input = new java.util.Scanner(System.in);

        System.out.println("Give height");
        int height = input.nextInt();

        char [][] array = new char[height][height];
        // ...

AFTER:

    public static void tehtava7(int height) {
        //                      ^--- NOTICE: NEW PARAMETER

        char [][] array = new char[height][height];
        // ...

Next, determine how you would like to "import" this.

Import by using an array of heights:

BEFORE:

    public static void main(String[] args) {
        tehtava7();
    }

AFTER:

    public static void main(String[] args) {
        int[] heights = new int[] { 2, 4, 6, 8, 10 }; // heights to import
        for(int height : heights) {
            tehtava7(height);
            //       ^---- NOTICE: LOOP AGAINST PARAMETERS
        }
    }

... or import parameters directly from command line:

   public static void main(String[] args) {
        // args = command line options

        // loop over command line options, convert to integer
        int[] heights = new int[args.length];
        for(int i = 0; i < args.length; ++i) {
            heights[i] = Integer.parseInt(args[i]);
        }

        for(int height : heights) {
            tehtava7(height);
            //       ^---- NOTICE: LOOP AGAINST PARAMETERS
        }
    }

java MyClass 2 4 6 8

output:

XX
XX
X  X
XX X
X XX
X  X
X    X
XX   X
X X  X
X  X X
X   XX
X    X
X      X
XX     X
X X    X
X  X   X
X   X  X
X    X X
X     XX
X      X

From what I understand your program is working correctly but you want to separate the logic of reading the height from the user from the method tehtava7()

So, basically you need to move that logic to the main method:

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    System.out.print("Write the height: ");
    int height = scanner.nextInt();
}

Then you can use that height to pass it as a parameter to tehtava7(...), so your method signature would become:

public static void tehtava7(int height) {
    // Your code here
}

But now, you want to return the 2D char array from it, so we need to change void for char[][] and add a return at the end, so in the end tehtava7 would end up as:

public static char[][] tehtava7(int height) {
    char[][] array = new char[height][height];

    for (int row = 0; row < array.length; row++) {
        for (int col = 0; col < array[row].length; col++) {
            if (row == col || col == 0 || col == height - 1) {
                array[row][col] = 'X';
            } else {
                array[row][col] = ' ';
            }
        }
    }
    return array;
}

And now you can use it on the main method like this:

tehtava7(height);

But as it returns a char[][] and you still need to print the N pattern you can do it this way:

printTwoDimCharArray(tehtava7(height));

And in the end, your whole program would look like this:

import java.util.Scanner;

public class PatternSample {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Write the height: ");
        int height = scanner.nextInt();
        /*
        Or you could make the call to tehtava7 and store it 
        in a 2D char array, and then use that as a parameter 
        for the print method like this:
        char[][] tehtavaArray = tehtava7(height);
        printTwoDimCharArray(tehtavaArray);
        */
        printTwoDimCharArray(tehtava7(height));
    }

    private static char[][] tehtava7(int height) {
        char[][] array = new char[height][height];

        for (int row = 0; row < array.length; row++) {
            for (int col = 0; col < array[row].length; col++) {
                if (row == col || col == 0 || col == height - 1) {
                    array[row][col] = 'X';
                } else {
                    array[row][col] = ' ';
                }
            }
        }
        return array;
    }

    private static void printTwoDimCharArray(char[][] myarray) {
        for (int i = 0; i < myarray.length; i++) {
            for (int j = 0; j < myarray[i].length; j++) {
                System.out.print(myarray[i][j]);
            }
            System.out.println();
        }
    }
}

And the output:

Write the height: 7
X     X
XX    X
X X   X
X  X  X
X   X X
X    XX
X     X

I must say, congratulations, you were almost there, keep the good work :)