I need to assign the numbers 1-26 to the letters a-z.
Why can’t I type Alphabet[]=Range[26] ?
Thank you so much for your help! 
Try with
Evaluate[ToExpression /@ Alphabet[]] = Range[26]
For some background, the individual letters in the Alphabet[] function are strings. A string cannot be set equal to another object, that is why Gianluca made use of the ToExpression command.
If you typed out {a,b,c…} = Range[26] it would work for the individual letters, or as Gianluca did, the ToExpression command accomplishes this.
Thank you, already helped me out a lot, but I immediately stumbled upon the next problem when trying to delete the assignment again: I tried it writing
Evaluate[Alphabet[]] =.
but if I am right, Mathematica now works with the values I assigned to the variables, and not the variables themselves. That’s why I get:
In[79]:= Evaluate[Alphabet[]] =.
During evaluation of In[79]:= Unset::usraw: Cannot unset raw object a. >>
During evaluation of In[79]:= Unset::usraw: Cannot unset raw object b. >>
During evaluation of In[79]:= Unset::usraw: Cannot unset raw object c. >>
During evaluation of In[79]:= General::stop: Further output of Unset::usraw will be suppressed during this calculation. >>
Out[79]= {$Failed, $Failed, $Failed, $Failed, $Failed, $Failed,
$Failed, $Failed, $Failed, $Failed, $Failed, $Failed, $Failed,
$Failed, $Failed, $Failed, $Failed, $Failed, $Failed, $Failed,
$Failed, $Failed, $Failed, $Failed, $Failed, $Failed}
Is there a way to make variables stay symbolic for a specific calculation, even if I assigned a value to it?
Are there any other ways to achieve what I am trying to do?
Thanks again!
Thread[ToExpression /@ Alphabet == Range@26]
{a == 1, b == 2, c == 3, d == 4, e == 5, f == 6, g == 7, h == 8,
i == 9, j == 10, k == 11, l == 12, m == 13, n == 14, o == 15,
p == 16, q == 17, r == 18, s == 19, t == 20, u == 21, v == 22,
w == 23, x == 24, y == 25, z == 26}
Maybe this one you are looking for?