On the first computer, File -> Backup Wallet, then on the machine you want to move your wallet to, you have to find you BiblepayCore folder, which holds the blockchain, wallet, configuration files... The default location is C:\Users\{username}\AppData\Roaming\BiblepayCore. In this folder, replace the old wallet.dat with the backed up one (you might want to copy the old one somewhere just in case). You should do this while your biblepay is closed, and after copying the new wallet.dat you should run biblepay and everything should work.