how to upgrade AMD Sempron 140 processor on Asus MOTHER board 3 GB RAMModel M2N68-AM SE2

Here is the list of supported processors for that motherboard, along with the required BIOS update needed for the CPUs and the board revision.
https://www.asus.com/Motherboards/M2N68AM/HelpDesk_CPU/

Good luck. They are all long ago obsolete and will need to be purchased used on someplace like ebay.