With scaled y ticks=false, the y-axis tick labels will be 1 \cdot 10^6, 2 \cdot 10^6, and 3 \cdot 10^6.
To configure the number format, you can add /pgf/number format/... options to tick label style={...}. For example,
tick label style={font=\tiny, /pgf/number format/fixed}
will change y-axis tick labels to 1,000,000, 2,000,000, and 3,000,000.
For more info, you can check pgfplots package manual, sec. 4.15.3 "Tick Scaling – Common Factors In Ticks" and sec. 4.13 "Number Formatting Options".
\documentclass[notheorems]{beamer}
\usepackage[english]{babel}
\usepackage{tikz}
\usepackage{pgfplots}
\usepackage{biblatex}
\begin{document}
\begin{frame}
\begin{figure}
\centering
\begin{tikzpicture}
\begin{axis}[
% title={Dengue reported cases from 1980 to 2022},
scaled ticks=false,
xlabel={Year},
ylabel={Cases per million},
xmin=1980, xmax=2022,
ymin=0, ymax=3300000,
xtick={1980, 1983, 1986, 1989, 1992, 1995, 1998, 2001, 2004, 2007, 2010, 2013, 2016, 2019, 2022},
xticklabels={1980, 1983, 1986, 1989, 1992, 1995, 1998, 2001, 2004, 2007, 2010, 2013, 2016, 2019, 2022},
legend pos=north west,
label style={font=\tiny},
legend style={font=\tiny},
tick label style={font=\tiny, /pgf/number format/fixed},
width=10cm,
height=6cm
]
\addplot[
color=blue,
mark=x,
]
plot coordinates {
(1980, 65523)
(1981, 377916)
(1982, 68892)
(1983, 40544)
(1984, 38904)
(1985, 158193)
(1986, 88093)
(1987, 134013)
(1988, 467386)
(1989, 94179)
(1990, 157662)
(1991, 254749)
(1992, 112567)
(1993, 98598)
(1994, 232051)
(1995, 331417)
(1996, 287519)
(1997, 410392)
(1998, 729425)
(1999, 317158)
(2000, 394857)
(2001, 636977)
(2002, 1001073)
(2003, 506726)
(2004, 257251)
(2005, 413122)
(2006, 537412)
(2007, 874750)
(2008, 884334)
(2009, 1099742)
(2010, 1648569)
(2011, 1073990)
(2012, 1164366)
(2013, 2384803)
(2014, 1184045)
(2015, 2416018)
(2016, 2175409)
(2017, 579027)
(2018, 561689)
(2019, 3190778)
(2020, 2326115)
(2021, 1254648)
(2022, 211882)
};
\addlegendentry{International}
\addplot[
color=red,
mark=x,
]
plot coordinates {
(1980, 9003)
(1981, 10861)
(1982, 19034)
(1983, 12925)
(1984, 7560)
(1985, 82273)
(1986, 55248)
(1987, 108955)
(1988, 441382)
(1989, 65803)
(1990, 114431)
(1991, 217077)
(1992, 72319)
(1993, 41250)
(1994, 134342)
(1995, 211396)
(1996, 226960)
(1997, 307625)
(1998, 632804)
(1999, 252762)
(2000, 326846)
(2001, 572876)
(2002, 903891)
(2003, 434264)
(2004, 193334)
(2005, 294943)
(2006, 432661)
(2007, 724077)
(2008, 810134)
(2009, 756755)
(2010, 1317801)
(2011, 923793)
(2012, 843840)
(2013, 1857228)
(2014, 857983)
(2015, 1978921)
(2016, 1860466)
(2017, 385076)
(2018, 382243)
(2019, 2453060)
(2020, 2021272)
(2021, 1134555)
(2022, 199319)
};
\addlegendentry{South america}
\addplot[
color=yellow,
mark=x,
]
plot coordinates {
(1980, 0)
(1981, 0)
(1982, 12000)
(1983, 0)
(1984, 0)
(1985, 0)
(1986, 47367)
(1987, 89393)
(1988, 190)
(1989, 5334)
(1990, 40642)
(1991, 97209)
(1992, 3501)
(1993, 6915)
(1994, 54453)
(1995, 124775)
(1996, 175749)
(1997, 254074)
(1998, 535283)
(1999, 204131)
(2000, 231412)
(2001, 412388)
(2002, 778037)
(2003, 341189)
(2004, 112851)
(2005, 203356)
(2006, 345922)
(2007, 558413)
(2008, 724427)
(2009, 520660)
(2010, 994158)
(2011, 753487)
(2012, 597450)
(2013, 1473645)
(2014, 591080)
(2015, 1649008)
(2016, 1500535)
(2017, 252054)
(2018, 265934)
(2019, 2248570)
(2020, 1467142)
(2021, 975474)
(2022, 167602)
};
\addlegendentry{Brazil}
\end{axis}
\end{tikzpicture}
% \caption{Dengue reported cases from 1980 to 2022~\myfootcite{PAHODengueData2022}.}
\end{figure}
\end{frame}
\end{document}
Note it's recommended to provide the complete example directly to this site, so the Q&A could be self-contained even if the link is dead.
