I followed Dat’s tips:
“Sys A: DFU mode”
“Sys B: Bootloader mode”
“Sys D: switch to serial mode.”
I bought two UC5550 UCM boards at the same order from MOUSER. One board has WIFI and the other without WIFI.
The UCM without WIFI could be successfully uploaded to boot loader V2.0.5 and its USB interface UC5550_UC5550 shows on Debug side.
However, the UCM with WIFI remained at V2.0.4 after successful “Update to Latest” even re-tried several times.
I tried to upload the boot loader with DfuSeDemo with file “UC5550 Bootloader v2.0.5.dfu” and "Upload successful. However, the boot loader version keeps on V2.0.4.