You won't be able to update the exact map that is in bnet as each map reserves it's name forever and is forever bound to account that uploaded it.
If original owners give you mapfile then you can edit it and upload your separate version. Same if they left it unlocked in bnet (making it available to be downloaded by everyone trough editor, tho in 90% of cases people lock their maps). If they can't be reached or don't want to give you the file it's probably better to remake the thing from a scratch.
There are ways to steal maps, but that removes your ability to edit triggers trough GUI, and well... other people's code is usually hell and without knowing why and how original authors made their maps you'll just go insane as changing one thing will usually make other things break and you'll end up being stuck in infinite loop of trying to find out how and why one unrelated thing breaks so many others.