^^ What they all said
You have to practice, everyday if you can, singing your notes from low to high. Don't need to use words, just a vowel would do-- I suggest the vowels "ooo" and "ahh".
Find the highest note you can hit, and don't push yourself further from there, just sing the notes back and forth, low to high to low. After some time of practice, when the highest note is easier to hit, then bring up your highest note and practice till there the same technique, so on and so forth.
ps.
Just a note that in between these low-high-low practices, you might come across a 'bridge'; This is the point where you can feel the way you sing the notes changes. Like your throat sorta like.. disengages, or change gears, to put in layman's terms.
It's called the passagio, and it's a normal obstruction to singing low-high-low scales. Don't be discouraged if it gets in the way..
It would be helpful to be aware of where that happens, like which note etc, and try to concentrate on getting a smooth 'change' between the high and low notes. The benefit to this is, besides getting a smoother, fuller vocal range, your high notes will be more developed in terms of tone because the transition there would help 'arrange' your vocal gears to support those notes.
I'm explaining this in the most layman's terms as I can, I hope you understand what the hell I'm saying lol..