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This video might be used as a vague guide: it shows that you might flip over the keyboard, then remove rubber or plastic nubbins that cover the case screws, then remove the case screws, and take off the whole bottom case as one.
If you need more specific repair/disassembly instructions guidelines or diagrams, a Yamaha service manual for your specific model would be very helpful. Would you be interested in a choice of several sources for such a service manual? if so, let me know, and I will research and list such sources.