For Capital A to Z
Put A in first cell and drag down.
If you want to do it through formula –
In Cell A1, put this formula and drag it till 26th row
=CHAR(65+ROW()-1) or CHAR(64+ROW())
CHAR(65) is A. You can verify this by CODE("A") formula which will generate 65.
If you are putting this formula is some other row apart from 1, adjust the mathematical expression within CHAR in such a manner that first entry is 65.
For example, if you are starting in row 10, the formula would become
= CHAR(65+ROW()-10) or CHAR(64+ROW()-9)
For Small a to z
Put a in first cell and drag down.
If you want to do it through formula –
Replace 65 with 97 or 64 with 96 in above formulas.