Hi
we upgraded succesfully from 7.1.1 to 7.5.4.2 as per installation instruction
after then we are faced problems.
First we saw our existing Board Rules has beed overwritten

Then when i logged in as admin and opened control panel
i can see all the control panel informatoion
but i am not able to update primary setting,Registration setting, on each page after submitting gets error page not found.

publec site works fine, user can post etc

Pls help! we can not revert back to older version

also when my opens thsi pages its gives error
ubbthreads.php?ubb=showprofile&User=397

error:- php has encountered accees violation

Sounds like you may have a few file permission issues.
Damage is already done with the board rules I believe.
But the file is forums/includes/boardrules.php

It may take a little time but follow the documentation for file/folder permission settings. Especially all the new files that were not part of your original version before upgrading.

pls let me know where do i get file/folder permission settings

GOTO
http://www.ubbcentral.com/doc_install.php
Look for file folder permissions

You can also check your permissions at:
Control Panel » Permission Checks.
Anything that does not show OK. Click on it and it will show all of the files and which ones are bad.

Looking at this a little closer. You may have missed a note on upgrading.
From the upgrade documentation.

Upload all files and directories from the unzipped UBB™ directory to the UBB™ installation directory, except for the includes, styles, and cache_builders/custom directories. You will want to upload all of the .php files within the cache_builders directory, just make sure not to upload the custom directory or files within it.

If this is the case. If you have a backup of the site before upgrading.
Upload your backup files to includes,styles and cache_builders/custom directories only.

Sounds like you overwrote the includes/ directory, which would have cleared out your board rules and configuration files..

before ANY upgrade i do, the 1st thing is to backup the site to the local HDD for this type of issue

i exactly did as per instruction in installation doc
i have two server one test and one live
on test i re[peat several timesthe process and facing same isssue in each time