If you really want to rename your user account you can try this - I did it and it works OK. May be easier to just make a new account called "Nick" and then sign on as "Nick" and delete "Acer" though.
If you want to risk it you can try this (it worked for me). To rename ACER to NICK
User Profile Folder - Change User Account Folder Name - Windows 7 Help Forums
1. Backup
2. Create a new local admin user called TEMP
3. Sign on as TEMP
4. Rename user folder ACER to NICK
5. Replace all references to ACER with NICK in registry. I had about 50 if I remember correctly.
6. Edit configuration settings files in C:\Users\NICK\AppData\Local\Microsoft\SkyDrive\Settings\ and replace ACER with NICK where found.
7. Reboot and sign on as NICK
8. Delete all libraries from file explorer and then recreate default libraries (right click)
9. Delete TEMP user.
10. Delete and recreate all fingerprint data if you use it.
You may have to update default save locations etc depending what other software you have installed.
This worked for me - I renamed the generated id SALOU_000 on my machine with SALOUA and it works perfectly. Depending what software is installed you may have issues I did not though. If you aren't comfortable doing this you could just create a new user and delete the old one.