You can delete the old version of the Lightroom from your computer. Make sure your working catalog is safe.
You can directly install Lightroom on the external drive,but adobe does not recommend that.
Oh, thank you! Just to confirm: if I leave just the LR5 program on the computer I can launch the catalogs from the externals - but I can delete the earlier LR software - is that correct?
Sent from my iPhone so my apologies for typos and aberrant spelling.
If you are keeping 5.7 on your machine you should first make certain it will open the latest updated catalog. That is, if you see something like Lightroom_misc.lrcat and Lightroom_misc-2.lrcat, where the second one came from Lightroom 6 or later, make certain version 5.7 will open that second catalog before deleting Lightroom_misc.lrcat.
Thank you both for your help. I feel like an idiot but I'm still not clear. If I try to open the older versions (LR 2 and 4) to make sure all those images have been incorporated into my current catalog, I get a message saying I have to update to LR 5 in order to open . I AM using 5.7 and it does open the current catalog. My working catalog, on the external drive would be disconnected before I delete anything on the computer.