That tutorial by UP~Boomer is very instructive and useful. Besides deleting, rotating and adding objects you can do basically everything you want to do with the map:
Changing heights, pasting textures, adding railways, bridges, changing colour of water.
Remember: when saving the map click
SAVE and not
SAVE AS.
It is recommended saving the map during the mapmaking procedure. Just in case you made a huge mistake, pressing EXIT button by accident or power outage, it saves you a lot of time in redoing things. And copy paste the changed file(s) somewhere else. We can not say it enough: backup, backup, backup.
Probably you have already seen this thread, a list full of mapmaking utilities.
https://www.sas1946.com/main/index.php/topic,33463.0.html Don't forget to read KevinP's tutorial about surf and shallow waters and how to change colour of water.
Zuti's Actors Management is also useful. For instance, in case you decide to delete a particular object which has been added already a 100 times all over the map.
Instead of scrolling the whole map to delete them his Actors tool let you do it with a single entry.
By the way, your retextured Finnish Gulf map looks very promissing.

Glad to see you want to get rid of the 'old skool' city textures.