You cannot install Evolution X like a regular app. You need a PC and patience.
Evolution X is a custom Android ROM focused on Pixel-like features, theming, and performance. For the Redmi 6A (Mediatek MT6761/Helio A22, codename often "cactus" or "lancelot" depending on region), official Evolution X builds are not widely available or maintained; only unofficial/experimental ports exist. Installing such builds may require unlocked bootloader, a custom recovery that supports the device, and patched kernels compatible with Helio A22. evolution x redmi 6a