A dress blue uniform is basically a suit. As such, it can be tailored like a suit would be tailored. Each piece needs to be tailored separately. Before proceeding, each upper body piece must already fit in the shoulders and the pants must fit in the waist. If these conditions are not met, you're better off buying a new uniform.
The dress shirt
Put on your shirt. Use pins to mark extra fabric from the sides of the shirt and from the sleeves.
Remove the shirt and mark the inside with chalk.
Invert the shirt and re-pin it on the inside along the chalk lines.
Sew just outside the pins, or along the chalk lines. Cut away the excess fabric and turn the shirt right-side out. The shirt is now tailored.
If you're hemming the pants, measure the amount you'll need to take in.
Fold the bottom of the pants in by that much. The hems should be done by hand from the inside out with matching color thread. Use a blind hem stitch, essentially only piercing the top fabric at distant intervals and only taking a very small amount of fabric.
Turn the pants right side out.
Remove the lining. This can be done using the seam ripper along the lining's seams.
Turn the jacket inside out and put it on the wearer. Close the buttons and pin off the extra fabric.
Sew outside the pins and cut off the excess fabric. Reattach the lining if it's necessary.
Most alterations of official uniforms should be done by experienced tailors to comply with military regulations.