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You should always back-up your site and datablase before performing an upgrade.
If you follow the upgrade instructions very carefully you should not loose any data. But also, if I were you, I would upgrade from 2.0.13.1 to 2.0.13.2 before I went to the 2.0.14RC-1 version.
During the upgrade the files which have necessary changes will be uploaded and should overite those which are already installed. But like it says, follow the upgrade procedures carefully.
Quest