In general, you should first make your compiler toolchain into a PlatformIO package by adding a package.json in its root directory. Its content can be copied from the current <user directory>/.platformio/packages/toolchain-atmelavr, but with at least modified version field.
You can then make PlatformIO use that package by setting it as the sourec for the toolchain-atmelavr package. As is documented.
platform_packages =
toolchain-atmelavr@symlink://C:/Users/Max/Desktop/better-avr-toolchain
This is a project-level change, but if you want all projects to use it automatically, you need to make a change in the platform, that is <user directory>/.platformio/platforms/atmelavr (or atmelmegaavr), where you can modify the source for the package in the platform.json, by modifying the version field for the toolchain-atmelavr package. (See current)
You can also read more details here.