Get the .img.gz file corresponding to your device (e.g., S905, S922X) from the GitHub Release page.
Version 3.8 is often cited as the "stock" version found on many pre-loaded Anbernic devices (like the RG351P). Many users in the community eventually transitioned to custom firmwares like 351ELEC for better ongoing support beyond version 3.8. Error found in version 3.8 (s905w and s905x3) #312 - GitHub emuelec 38 top