What is blend1 file
A BLEND1 file is a backup file created automatically by Blender. It saves a copy of the previously saved version of the currently opened BLEND file and is used to recover a BLEND file’s prior version.
How do they work?
Generally, whenever you save, this file gets overwritten. If you have blend1 files enabled before overwriting, the old filename extension is renamed to blend1. And if blend1 already exists, it becomes blend2, and blend2 becomes blend3, and so on until you reach the number of copies you ordered Blender to preserve (the limit is 32 saved versions).
How to Open it
Change “project.blend1” to “project.blend” to open the previous version of your file.