It is best to delete the data files and then rebuild the blockchain. The easiest way to do this is to choose the debug screen in the Menu/help and then take console tab. At the bottom of this tab you can type resetdatadirfull followed by an enter/return. see the picture.
After this the wallet will shut itself down and you will have to restart it manually. The blockchain will then be rebuilt. this will take 45-150 minutes, so be patient for a while.
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