Not really necessary usually, but it helps. Just delete the blocks and chainstate folders.
You don't need to download a new client unless there's an update. Just close it, delete those folders, and relaunch.
thanks, i was using some other guys client with improved pps, however it seems to not be mining the correct chain. Unless orphans are everywhere now, some had 20+ confirms before turning to a question mark
going to delete all the stuff in the roaming folder except the wallet hopefully get back to mining properly.