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You can have Gantry 4 and Gantry 5 installed simultaneously but there is no way to "upgrade" a Gantry 4 site to Gantry 5; they are completely different code bases.
Start with installing Gantry 5 + whatever RocketTheme you want. From there, content loaded in articles will still output similarly. For content inside of modules you can either change which Module Position the module is using, to use one from your new theme, or you can copy+paste your content in Gantry 5 Particles.
If you're new to Gantry 5 please read the first 3 chapters here to get the basics:
docs.gantry.org/
SEARCH the forum first! These boards are rich in knowledge and vast in topics. This includes searching just the 'Solved' forums, using Google, and using ChatGPT