Install Fonts on Shopify Store

I need assistance with installing new fonts on my existing Shopify site. Shopify appears to have removed from the library the fonts I used to use (Optima) and my site (leocadogan.com) has now defaulted to a font I don't want. I have uploaded the Optima fonts to the back-end and they now need to be coded in. There are YouTube videos on how to do this and I think it will be easy for the right person (I am not that person!)