There are a lot of unofficial Windows Mobile 6.5 ROMs for Mio A702 PocketPc phone but I recommend the one created by
Ansar with 8Mb PagePool size.
After downloading, unzip it. Along with a lot of file, you will find a large .bin file. That is actually the ROM image file which can be flashed with the
ROM Update Utility. You must first enter Bootloader in your Mio PocketPC phone by pressing and holding both Volume Up and Power buttons together until it powers on and when you see the light, release both the buttons and then press down navigation button to goto Rom Upgrade menu. Choose the Rom Upgrade mode via CE Image on USB and then connect to PC. Now use the ROM Update utility to flash the downloaded .bin ROM image on your Mio PocketPc.
Tell us how it goes.