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Not a stupid question if you haven't done it before...there are FAQ's on upgrading versions the link to tutorials on it if you haven't done it before. Yes...you delete the install file because XOOPS is already installed - you don't need it.
2.0.14 was 'different' because it didn't have an upgrade file (and that was discussed on the original news post of it's releas) We can thank core dev for giving us an upgrade file 2.0.14-2.0.15- nice and easy