how do i upgrade my maturity level?

[Cherubela Constantine]

where are the settings to change maturity level to Adult

 

[Rolig Loon]

In three places:

1.  Preferences >>> General

2.  Search >>> Upper left corner

3.  Marketplace >>> Top/Center

Depending on what you want to have Adult access to, you may set any of those or all three.  If you are at least 18 years old in RL, the change should be immediate.

ETA:  Even after you have changed your rating to Adult, you may find that you are still denied access to many Adult regions in SL.  Don't take it personally.  The most annoyingly childish griefers usually get bored or banned within their first month in SL, so landowners in Adult regions typically block entry to anyone who is less than a month old.  It's just a business precaution.