Wren Wiki help
The pre-release is a single executable file, and it assumes that a full Wren installation was performed before.
To get the pre-release you need to download the executable, and then copy that over the existing wren.exe.
You may want to rename the existing wren.exe before you do this, to have an easy way back (for in case there are unexpected issues with the pre-release).
You can download the pre-release using this link.
And the details for it can be seen here.