When you purchase an item, your Points are automatically exchanged for Coins at a rate of 1 Coin per 2 Points.
Note: Your Points will not be exchanged if you've already purchased enough Coins to buy an item.
The Point exchange rate may differ depending on the country you're using LINE in. You can view the various rates here.
The Coins you've purchased and the amount of Coins that can be exchanged from your current Point balance will appear in your Coin balance. If you want to check your Coin balance, see here.