T-Mobile has become the last US carrier to release the Android Oreo update for the LG G6. Build H87220a rolled out to a small group of users last week but has now begun rolling out broadly to T-Mobile customers with the 2017 flagship phone.
To check for the update, go to Settings>About Phone>Update Center>System Update and check for the update there. Keep in mind that it is still rolling out so it could well take another day or two before it lands on your phone.
T-Mobile, who generally is one of the first to release software updates, has taken their time on this one. AT&T, Sprint, and Verizon have all released the update, in some cases nearly two months ago, so it is unclear why it took T-Mobile so long.