Update Shopify Theme

I have a Shopify website that was set up 7 years ago. I had an in-house employee that was proficient in helping me set this up and customizing it to the best of her ability. She is no longer with me, and I need some customization help. Need to develop a new theme on my Shopify store. Apparently, it is too old and some of my customers are having trouble checking out. Shopify says it’s due to the age of my theme.