Installing fresh will be the quickest resolve, in this instance, and choosing UEFI instead of MBR is definitely recommended. A shiny brand new SSD might also be worth considering, though I won’t presume to insist that it’s needed.
If using the existing SSD, make sure it’s ready for a UEFI OS; by destroying the existing partition table (MBR) and creating a new one (GPT). Otherwise, Manjaro might continue to install an MBR based system (again). This is usually handled during install, but be mindful, nonetheless.
You should also choose UEFI when creating the USB installer, otherwise, an MBR USB will likewise force an MBR installation.
The first of the following guides describes how to manually partition your drives im preparation for a UEFI install. The only addition I will make is to suggest creating a separate /home partition, which will allow much easier system recovery in future without sacrificing your user profile.