Welcome @Atahualpa76. The upright version that is shown in the instructions is described with the optics module from the ‘low cost’ version of the microscope. That module has no objective lens, the lens from the Pi camera is used with a spacer to give magnification and resolution similar to a microscope using a 20x objective.
If you want to use an objective then the optics module that is described in the ‘high resolution’ version of the microscope will also fit on the upright, the optics modules are interchangeable. I currently have an upright version set up with a 40x objective. You might need a longer camera cable, I think I am using 450mm.
If you are building a microscope right now I would recommend using the current master version, rather than the v7.0.0-beta1 release that is linked from the web site. I would not usually recommend a development version, but there are a large number of bug fixes and improvements since beta1, particularly for the upright version. We are approaching the next beta release so there will not be breaking changes in the current master until that beta2 is released (unless there is some mistake Edit 2024-5-17: Beta 2 is now released, so this no longer applies.
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If you are choosing between upright and the standard version, the main consideration is what kind of samples you are using. The Openflexure standard (inverted) microscopes are more compact and slightly more stable because the optics are in the body. The upright is really intended for samples where it is not possible to turn them upside down to use an inverted microscope. However there is not really any performance difference.
If in doubt, I would build one of the inverted versions as they are more tested and the illustrations in the instructions are more comprehensive. You will not really loose anything if you then change your mind, all of the electronic components and most of the printed parts and hardware are the same. From memory to change an inverted to an upright you would need to print a new illumination module and platform, a z-spacer and a separate z-axis and an extra small and large gear and z-foot. You would need one more o-ring and a few more screws. You might need another condenser lens if you don’t want to push the lens out of the condenser to put into the new one, and a longer camera ribbon cable. So that is a few spares of the hardware and printing 7 small pieces. In the image in the first post on the thread the two green parts are new for the upright version, basically everything else would be part of a standard inverted build.