Press
to view the calculus menu, then
to select the second derivative option, der2.

The syntax is: der2( expression,variable
[,value] )

The second derivative option algebraically finds the second derivative
of expression with respect to variable at value.
If value is omitted, the current value of variable is used.

Note: Unfortunately, the TI cannot differentiate some functions
algebraically. For these functions use the less accurate
numerical differentiation option nDer twice.