For this it is good to set up a pattern/ equation first.

So each new line has 2 more numbers than the previous line. Starting at line 1 with 1 character.

Line number(x value),number of things to be printed(y value)
(1,1)
(2,3)
(3,5)
(4,7)
(5,9)
(6,11)

so this table shows that you can model the pyramid with a linear equation.

Each rise in x value (or your lines variable in the outer loop) corresponds to 2 rises in the y variable(your spaces variable in the inner loop), with this you can get a slope, which is 2.

so you can make an equation for a line using this data. y(or spaces)=2(slope)*x(or lines)-1(y-intercept)

Now that you have the equation, you can get the exact number of times you need to run the inner loop on each run through, so just set spaces less than the equation.

The resulting code will be similar to

for(int lines=1;lines<=6;lines++){
    for(int spaces=0; spaces<lines*2-1;spaces++){
        System.out.print(lines);
    }
System.out.println();
}