At present you have a temporary stay visa so you will have to apply for a Certificate of Eligibility. The most appropriate visa is a Cultural Activities visa. If the certificate is issued during your stay, you can use it when applying to change your visa status. In case the certificate is not issued during your stay in Japan, you will have to leave the country and ask them to send the certificate overseas. After receiving it you should then apply for the visa at the consulate or embassy of the country you are staying in, then re-enter Japan.

The followings are notes to help you obtain a Cultural Activities visa.
1. The flower arrangement teacher must be a high level expert. Documents to prove his achievements are required. If his school belongs to a corporation, a corporate registration copy is required.

2. The father of your friend is required to provide proof of employment and tax information. You are required to provide a certificate showing the balance of your bank account. In this way you need to prove that you have no problems staying in Japan.