Here's a quick tip for accessing your upper register with ease. Begin by blowing air though a straw (so that all the air leaves through the end of the straw). Do the same thing and add a voiced glide from low to high. Observe the amount of space and lift in the back of your mouth.
Now glide through the straw again using a line of your go-to song.
Next, remove the straw and sing your song melody on the word "COO". Each word should be replaced with a "hooty" COO - think like an owl.
Lastly, sing your normal words. The key is to maintain to feeling in the straw and hooty coo.
This will help access your upper register and keeping the phonation flowy and easy!
STEP 1. Straw Sirens
STEP 2. Straw Voicing on Melody
STEP 3. "Hooty Coo's" on Melody
STEP 4. Insert normal words