[clayto2]

Is there any reason why I cannot or should not update my Samsung Tab A from 6 Marshmallow to a later version --- 7+. If I can which and how? I want to use the new Windows 10 My Phone app. It is of course not a phone, although Android often seems unable to detect it is a Tablet.

[SpookDroid]

Unless the manufacturer has released an official version for your specific model and region, nope, you can't. You can, however, find custom ROMs that are based on later Android OS versions as long as they are created for your device model. These require a little more handling and expertise than a simple 'UPDATE' button, though.

[Rukbat]

Check at https://www.sammobile.com/firmwares/, for the Tab A. If there's nothing there, do a google search for samsung tab a roms. (I doubt that you'll find an update, but you may find a ROM [if Samsung has written one] or a custom ROM [if someone at XDA has written one] for Nougat or something later [and don't discount LineageOS], but you'll have to do a full backup [see Backing up an Android Device] before flashing the ROM, then restore everything.)