To replace buttons on a blazer, remove the old ones by snipping the threads holding them in place, and then sew new ones in the same spot. Required tools include a needle, thread, buttons and scissors.
Use a small pair of scissors, such as the scissors or a knife on a pocket multi-tool, or a seam ripper to cut the threads holding the old button in place. Carefully remove the threads and the button to avoid damaging the blazer's fabric.
Choose a replacement button and thread that complements the blazer's color, and place the button where the old one sat. Thread the needle with the chosen thread, and sew the button on by sending the thread through both the fabric and the holes in the button. For two-hole buttons, simply sew in a loop, while buttons with four holes typically use an X pattern for the thread. Four-hole buttons also provide the freedom for alternate styles, such as parallel lines or a box pattern for the thread.
Keep enough room between the button and fabric for flexibility and easy buttoning during wear, and wrap extra thread around the length of thread between the button and fabric for a sturdy base that makes it easier to button during use. Securely tie off the thread to prevent unraveling.