This is a very easy fix.
The first thing you want to do is remove the back plate. This can be done with a prying tool. If you do not have a access to a prying tool you can actually use a credit card or an appropriate equivalent. If you stick the tool/card in between the screen and backing where the two come together you can work your way around the device, undoing the clips that hold it together. Once doing so be careful of not pulling out the speaker. Once you have the device open you can re-seat the buttons and put the backing back on.
For a more detailed description with pictures please refer to the troubleshooting page at LexiBook Tablet One Troubleshooting