Does a new software update take up more space, or replace the space of the old version? Just today my Galaxy s6 received the Android 7.0 update. I haven't installed it yet, though, because I'm concerned about space. Unfortunately the Galaxy s6 doesn't have external storage so I have to be very careful about managing my device. I have two questions:

1) How big is Android 7.0 for my phone?

2) Will Android 7.0 take up that amount in #1 PLUS what Android 6 takes up already, or will it replace what Android 6 takes up and hopefully leave me with very little decrease in storage?

Thank you!
System updates are applied (typically) to the /system partition, which is not accessible to you or your applications but is reserved for the operating system. It will over-write your existing Android version and should not take up more user space (this space is already reserved for the operating system, it usually is from 512MB to 4GB of reserved space, regardless if it is all used or not, and it is not accessible to you as a user). The exception being it needs to download the update file before it can apply it which can be quite large, anywhere from 200MB to 2.5GB in some cases but this is temporary storage only and it is released after the update is complete.

It is also important to note that due to the change in runtime of Android from 6 (ART) to 7 (JIT) the amount of space needed for your applications and they compiled versions reduces, sometimes drastically. It is not uncommon when upgrading from Marshmallow to Nougat to gain a few gigabytes of storage space after it is completed.