I cannnot download IOS 8 on my phone

I have a Spanish bank account and in order to use it for banking transactions they are asking me to download the Banco Sabadell App on my Iphone but it is saying that I need IOS8 to activate it but the app site wont let me do it. It is a very old phone. Please advise. I am 73 and not very good at all this

Toni Cooper