You can assign a value to a variable with the := operator. For example, to give the variable a the value of 4, you can enter
a := 4
Alternatively, you can use the => operator; when you use this operator, the value comes before the variable;
4 => a
The function sto or Store can also be used; again, the value comes before the variable
sto(4,a)
After any one of these commands, any time you use the variable a in an expression, it will be replaced by 4.