So you opened directly zip archive and not extracted (unziped) it. Please first extract it to separate catalog (in Windows right click on zip archive and extract or similar command). And next go to extracted catalog and open start.html.
You need export it first if you don't backuped it (backup should be first thing you do). Open old wallet, go to Account (top right corner) and there will be export wlt file option after you provide password. Next you can import this file to new wallet using import option. You can also import private key directly. Sign up > Private key wallet option, choose mainnet, new password and wallet name and provide private key from backup. Please keep private key backup in safe offline place. Same with wlt file.
Harvesting gives you possibility to harvest block. If you harvest block then all fees from block will be added to your account.Right now it's not much profitable but on new blockchain it will improve because there will be additional block rewards.