You can extend it, at the discretion of the police.
From Brazil Gringo:
Requests to extend your tourist visa for an additional 90 days must be
made at the local Federal Police office. A complete list of Policia
Federal locations is found at the bottom of this post.
It’s very important that you extend your stay before the initial 90
allotment has run out. If you stay 91 days and go to the Federal
Police expecting to get an extension the Federal Police may give you a
paper telling you that you have 7 days to leave the country instead.
You will need the following documents:
A filled out visa extension request form. Found here.
A printed bank
statement or valid credit card to prove you will have sufficient money
to spend another 90 days in the country; the bank statement has to be
in your name.
Proof of outbound travel. For simplicity’s sake, bring
an airline ticket. A bus ticket will probably work without too much
explaining. Having none of this, but having hotel bookings in a
neighboring country may work with a bit of explaining done in your
best Portuguese.
Your passport with the entrance stamp to Brazil.
Your
tourist card (Cartão de Entrada), issued when entering Brazil.
Receipt
of payment of visa fee. To do this you’ll first need to fill out this
online form (updated July 2018) and print out a receipt which you can
pay at any bank. On the web form under Unidade Arrecadadora choose the
city you’ll be submitting your request to. If you’re in a capital city
you’ll choose Superintendincia regional no estado de [whatever state
you’re in]. For codigo de receita STN use 140090, being the code for
visa extentions. Then click ‘gerar guia,’ print out the form and pay
the R$67 fee at the bank. Bring the receipt with you when you submit
your visa application. Note that the website doesn’t always work with
all browsers, so if it does something funny for you then try in a
different browser.
