Upgrading processor (K7SEM v3.0 Motherboard)
Unfortunately, you have not provided any details regarding current specs (PSU, type of SDRAM, etc.) to enable anyone to suggest a replacement motherboard.
For freeform discussion, I would agree that FixYa is probably not the best medium, e.g. in discussing potential replacements. Rather, if you have a specific question which has a specific answer, then this site would be better suited to your needs.
As to your original question, the best answer I can offer (personally) is that if your CPU is soldered to the board, you should complain to the vendor who sold it that way (that's crap), and replace the board with something suitable (provide more details).
If there's a belief it may not be soldered, the heatsink is probably attached via thermal paste, and is going to be a PITA to remove. Sometimes it will pull off with some force, though on at least one occasion, the solder joints beneath the socket gave before the thermal paste did, causing the heatsink, CPU, and socket to all come off the motherboard.