I just wanted to reply to make sure that the instructions are clear in this thread.
To use High Res textures with lr-mupen64plus (Libretro) and GlideN64 place you teture file in the following location.
/recalbox/share/bios/Mupen64plus/cache/NAME OF GAME_HIRESTEXTURES.htc
By default in Dragonblaze (6.x) this folder does not exist. You will need to create it (either through SSH or SCP/SFTP software). In future editions of RecalBox the folders will already exist.
Replace NAME OF GAME with the correct name (there are other tutorials out there to hep you identify this)
This is cASe SenSiTivE so please make sure the names is exactly correct.
once completed use the Libretro console (hotkey +B) and find the "Options" menu entry in the Quick Menu and find/enable "Use high resolution textures" you may alsy want to enable the high res alpha channel depending on your texture pack/game/etc. Then just restart the core (hotkey+A) and you should see the new textures.