effectively all you need to do is copy the database (normal windows copy)
decide which tables you want in your backend - there may be some that you need to keep in the front end
delete or rename the backend tables in your front end (so effectively your dbs would now report these tables as missing)
then use file/getexternaldata/linktables to locate the backend (other copy of the database) and link the tables - now your dbs finds the tables again, but this time its tables in the back end, identified by a little arrow
afterwards you can tidy this up, by deleting the bits in each half of the database that are now unneddede, but you dont have to do this - i n fact it's probably better not to do so, until you are happy with what you have just achieved
job done
this really is a 2 minute job
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lots of utilities here to automate the process of connecting the back end, (mainly for the ease of your users - but all they do is connect as shown above)