Upgrade, no. Convert, yes.
An ASA appliance with ASA software and a Firepower service module is essentially running two separate operating systems - think of them as two hosts connected via the internal backplane interface.
An ASA appliance running FTD is a single unified operating system.
To move from one to the other you need to reimage the appliance - there are instructions for doing so here:
https://www.cisco.com/c/en/us/td/docs/security/firepower/quick_start/reimage/asa-ftd-reimage.html
But there are also important distinctions and caveats to evaluate prior to making such a decision (capabilities, licensing, management model etc.). You should work with your Cisco partner or investigate these carefully first.
