Trouble updating Nuvi

Hy, got a Nuvi 58LMT and I have a bit of a trouble updating it to CN 2017.20 I have 2.21 GB free of 7.15 and the preinstaled map is CN 2016.10. I made a back up,the infamous .systemfolder included on my pc. I downloaded CN 2017.20 on my pc via GarminExpress and when I try to instal it with Mapinstal in the internal storage it says that the map wont fit. It shows in grey the preinstaled map,which has 3,68 gb and I cant deselect it. The new map is shown to have 3.98gb.
I thought the new map will be automaticly instaled over the old one, deleting it, but I guess I was wrong. I understand that gmapprom.img is the preloaded map, CN 2016.10 in my case. Cant I just delete it and install the newmap in its place? And if it comes down to deleting, what files and maps should I exactly delete, dont want to end up with a bricked device. Also, I dont know where is the foursquaremap instaled,logicaly I sholud delete the old one as well, to install the new one(there is no foursquare.img in the map folder, just a disk image file, D4935050A.img. I could just buy a microsd card, but why keep an outdated map in the nuvi and have a new one on a card,seems like waiste of space and money to me. A bit of help would be welcomed.
Here is a pic of the system folder

and of the nuvi with the preinstaled maps

ad of the map folder in system