It's a lot of work to install a M.2 blade SSD Apple designed the system to support. It also turns out Apple is the only supplier of the needed SSD that will work in your system.
With the said, you could swap out your current HD for a SSD but you'll run into an issue with the internal HD thermal sensor the HD has and the SSD does not have. OWC makes a special cable which you could use to make it work. But given the added costs and the limits of storage doing a straight SSD I would recommend going with a SSHD instead. Seagate makes one that will work in your system.
That way you're getting the best value and performance. Or, sell your current system and get one with a fusion drive setup