How can I upgrade this motherboard ???

I don't think you can. It looks like the jumpers will only let you go up to a 200MMX. If you can figure out how to get the multiplier to 3.5X you might be able to go up to a 233MMX, but that would probably be the most. If you were thinking about getting an AMD K6-2, they need a Front Side Bus of 100MHz, but your board only appears to go up to 66MHz. As well, if you did want to go faster, you would want to get PC100 SDRAM DIMM's, but from what I saw in the manual the board only supports SIMM's.

However, it does appear to be an ATX board, so you should be able to install a new motherboard into your case. If you want to go with AMD K6-2, I personally like the ASUS P5A motherboards. If you want to go Duron or TBird, you would need to pick a new Socket A board (ASUS A7V or Abit KT7 for example). One thing to check is that you have a large enough power supply if you go with the Duron or Thunderbird. They like 300W. You could also go Celeron or P3.