Red ball is the center of mass. Add more mass to the nose, or use Overload mod to reduce the mass in the back. Blue ball is the center of lift. Add more wings to the front or back to move it

@EternalDarkness how can I make the red ball go in front of the blue?

@JayAr yes. Make sure that the CoM (red ball) is in front of the CoL (blue ball), and that you have enough lift. CoT (yellow ball) shouldn't be too low or high. Also, use dead weight to balance a plane

@EternalDarkness tnx dude btw I have a question when I'm building planes it always tilting upward or sometime downward how can I fix it?

I recommend making shortcut of the "SimplePlanes" folder as you might need it often.

Go to your file explorer. Select the "View" menu, and check "Hidden items". Go to "This PC", open "Users", open the profile you are using, open "AppData", "LocalLow", "Jundroo", "SimplePlanes", "Aircraft Designs", and find the one you need. Open it with notepad, and find the part that you need.

@EternalDarkness where can I find it?

Ok. Then find the XML file of the build containing that part, find that part, and delete it there by removing all its data.

@EternalDarkness I already saved it

Open the XML file of the build, find that part, and delete it, or undo until it's back to the size you can work with