UNR.PREVIEW2 is based on Alpha 1.0.16_02, so normal Alpha 1.0.16_02 mappings will work.
Requirements:
UNR.PREVIEW2 client .jar file (click here for an already deobfuscated .jar file)
UNR.PREVIEW2 server .jar file (optional)
Recaf (not necessary, you should be able to save the .jar with the mappings directly through enigma)
Open Enigma, select the UNR.PREVIEW2 jar and the Alpha 1.0.16_02 mappings. Open the "(none)" folder in Obfuscated classes and make every class name start with "net/minecraft/src/" (obviously without the quotes)
Save the mappings and in Recaf open the UNR.PREVIEW2 jar. In the Mappings tab, select TinyV2 -> Obfuscated to Intermediate. Select the mapping file you saved.
Requirements:
Deobfuscated UNR.PREVIEW2 client .jar (downloadable here if you didn't follow the steps above)
Deobfuscated UNR.PREVIEW2 server .jar (optional)
Open RetroMCP and run the command "setup a1.0.16_02".
Rename the deobfuscated jar to "minecraft.jar" and copy it in the "jars" folder.
Rename the deobfuscated server jar to "minecraft_server.jar" and copy it in the "jars" folder.
In RetroMCP run the command "decompile".
Requirements:
UNR.PREVIEW2 .jar
Open RetroMCP and run the command "setup a1.0.16_02".
Rename the UNR.PREVIEW2 .jar to "minecraft.jar" and copy it in the "jars" folder.
Rename the UNR.PREVIEW2 server .jar to "minecraft_server.jar" and copy it in the "jars" folder.
In RetroMCP run the command "decompile".