The V6 (or "S" board) is locked. You cannot easily flash Marlin to it due to locked bootloaders. You must buy an MKS Robin Nano V3 as a drop-in replacement.
Performing a is the single most impactful upgrade you can perform that costs absolutely nothing. However, unlike an Ender 3, Tronxy machines can be finicky. A wrong flash can brick your board. tronxy xy-2 pro firmware update