If you have an expression like `a+b^2' on the stack and you wish to compute its value where b=3, you can simply store 3 in b and then press = to reevaluate the formula. This has the side-effect of leaving the stored value of 3 in b for future operations.
The s l (calc-let) command evaluates a formula under a
temporary assignment of a variable. It stores the value on the
top of the stack into the specified variable, then evaluates the
second-to-top stack entry, then restores the original value (or lack of one)
in the variable. Thus after ' a+b^2 RET 3 s l b RET,
the stack will contain the formula `a + 9'. The subsequent command
5 s l a RET will replace this formula with the number 14.
The variables `a' and `b' are not permanently affected in any way
by these commands.
The value on the top of the stack may be an equation or assignment, or a vector of equations or assignments, in which case the default will be analogous to the case of s t RET. See section Storing Variables.
Also, you can answer the variable-name prompt with an equation or assignment: s l b=3 RET is the same as storing 3 on the stack and typing s l b RET.
The a b (calc-substitute) command is another way to substitute
a variable with a value in a formula. It does an actual substitution
rather than temporarily assigning the variable and evaluating. For
example, letting n=2 in `f(n pi)' with a b will
produce `f(2 pi)', whereas s l would give `f(6.28)'
since the evaluation step will also evaluate pi.