What you are going to need:

1) BG3 Modder's Multitool
2) Norbyte's ExportTool
3) Any text editor of your choice

NOTE:
If this is your first time doing something like that, you must unpack game files using MM first.
Open MM, go to Utilities > Game File Operations > Unpack Game files and unpack Shared.pak.

If you want to replace it with Old School Tattoo guitar:

Open MM and press the "Search index" button.
Paste d4198661-d450-1005-7ac5-1ce32ed3462b to the search field to locate the RootTemplates file (screenshot).
Copy this file and paste it to SteamLibrary\steamapps\common\Baldurs Gate 3\Data\Public\Shared\RootTemplates (create the folder if it doesn't exist).
Open this file and replace VisualTemplate value d4198661-d450-1005-7ac5-1ce32ed3462b with eb2ea997-f178-4743-b53d-d7f8b46b7ea7 (screenshot). Use Ctrl + F to find it quicker.
Use ExportTool to convert the file (screenshot).
Done!

If you want to replace it with a different one:

This involves some extra steps.
First, you need to unpack this mod using MM.
Once it's done, go to UnpackedMods\BT_Guitars\Public\BT_Guitars\Content\Assets\Characters\[PAK]_Armor and open _meshes.lsf.lsx.
Change the MaterialID of the cinematic model to the MaterialID of the desired color variation (screenshot).
Once it's done, repack the mod and update it as usual.

To undo replacement, delete _merged.lsx from SteamLibrary\steamapps\common\Baldurs Gate 3\Data\Public\Shared\RootTemplates.
This can be safely done anytime.