Databases cannot be merged. Sorry. Your best bet is to choose the one with the most important data or the most data. Make that "MAIN" database. Then the remaining other databases can be accessed when needed by MAIN MENU -> DATABASE -> OPEN OTHER DATABASE
A few things to consider ....
You could manually re-enter data from the abandoned databases into the MAIN database. This could be useful in generating PLCs. However, if you already have PLCs that you are happy with then there is little benefit in this manual re-entry..
If the program has calculated PLCs in one database that is not the one that you have chosen as the MAIN one then you should enter the PLCs as surgeon constants in the MAIN database. You could do this in lieu of manually re-entering all the data which would obviously lead to the PLCs being calculated in the MAIN database.
Rename all the other non-MAIN databases (files ending in .hdf) something meaningful like DrJonesPre2012HicSoapData.hdf.
Please consider configuring networking so that you can access the same database from multiple computers.