"from" is not used in "UPDATE" statement.
OleDbCommand cmd = new OleDbCommand("Update Purchases SET '" + name_of_column + "' =" + new_val + " WHERE '" + name_of_column + "' =" + cellvalue, con);
The way you are concatenating user input instead of using
leaves your program susceptible to SQL Injection Attack. Using an SQL Injection Attack, a nefarious individual could erase or damage your database.