Plz help me with my 6510

[quvinh]

I just got a second-hand 6510 and I like its funtion. But sometime when I browse my phone book, it stops working for some seconds and then restart. Someone tell me that it's due to I press keys too fast. Is there any one have same problem with me and Can someone tell me how can resolve it. Is it better to upgrage to v. 4.21. How can I do it by mysefl or I must bring it to Nokia center?
Thanks for reading and answering my question