Not exactly sure what you want to do but perhaps:
public void Trasf() {
String value = Scegli.getText().toString();
// add two strings or numbers together in the .put method
cv.put(CTable.PREL, mI.getText().toString() + " " + oldString); // for string
cv.put(CTable.PREL, int_new + int_old); // adding numbers
SQLiteDatabase db = mHelper.getWritableDatabase();
db.update(CTable.TABLE_NAME, cv, CTable.NO + " = ?", new String[] { value });
db.close(); }
}