Remove the tube and set it aside, then pull the old tire off the rim. Inspect your rim tape. Buy a new rim strip if necessary. Inspect your tube. Check the sidewall of the new tire.
Some tires are directional.
Look for an arrow. Install the new tire halfway on the rim. Inflate your tube a small amount and install it evenly in the tire, starting with the valve. Using your hands this time, work the other side of the tire back onto the rim, being careful not to pinch the tube. Once you have done this, you may need to seat the valve stem - simply push up on the stem so that the tube sits properly in the mounted tire.