This type of question comes up quite often. I used to write wills for a living and here's what I always advise:
1. Get hold of copy of this book:
http://tinyurl.com/j6ma44e
It's in many public libraries but, at under £3 per copy (inc p&p) secondhand from Amazon, it won't break the bank to buy it. Don't worry if you find an older edition; while there have been changes to the intestacy laws, everything about will-writing remains the same.
2. Draft your own will based upon the information and advice in the book.
3. If you're 100% confident that you've got it right, go ahead and sign it (in the presence of two witnesses who aren't beneficiaries of the will); it will then be a valid will. If you've got any doubts about your draft, take it to a solicitor and get him/her to 'tidy it up' before you sign it.
4. Tell relevant people (such as the executors of your will and/or its main beneficiaries) where your will is kept, possibly providing them with photocopies of it. There is no central repository for wills.
Having got that general advice out of the way, I'll now tackle the specific point raised in your secondary post:
All that your friend needs to do is to type out a codicil to her existing will, as follows:
"This is the first codicil to the will dated [date of existing will] of me [your friend's name in full] of [your friend's address].
1. I amend my monetary gift to [name of niece] to be the sum of [new amount].
2. In all other respects I confirm my will.
Dated:
Signature:
Signed by [friend's full name] in our presence and then by us in hers:
[Space for signature of first witness]
[Full name and address of first witness]
[Space for signature of second witness]
[Full name and address of second witness]"
As soon as your friend has signed and dated the codicil, and the witnesses have counter-signed it, the revision to the existing will comes into force. (The codicil should, of course, be kept with the original will).