You have two options.
- Use Rice's Video Plugin 6.1.1+, which will dump and read textures through the enabling of options.
- Donate to the Project64 team to access all beta versions of Project64, including those that support retexturing games via Jabo's Direct3D 1.7 plugin.
Project64, 1964, Mupen64, Nemu64...doesn't matter which you use.