How to upgrade from 3.5 to 3.8.6?
 

How do I do the upgrade? Will I lose my current forum and all the posts? Any help would be great! Thanks...

There's upgrade scripts for as far back as the 3.0 series built in to 3.8.6, all you need to do is follow the standard upgrade procedures...

1) Turn off (close) your forums
2) Upload the new files, overwrite the old ones
3) Check and see if config.php has changed, it may have, if so re-fill out the new one
4) go you http://www.yourdomain.com/forums/install/upgrade.php
5) Follow instructions until the end
6) Delete install.php
7 Revert your templates
8) Re-open your forums

You won't lose any forums or posts or attachments or anything like that. You will probably lose a lot of style customizations because many (all?) templates will need to be reverted to the 'new' originals to work on 3.8.6.

Also FYI 3.8.7 is due out any week now, you may want to wait- or just upgrade again when it comes out, it will be a lot quicker/easier... (probably no templates will need reversion).

you can always do a backup of your template or skin . Then you can use a program like araxis merge to find the changes and add them back to your skin / template. I always do a backup of my skin before i upgrade and also i backup the database in case something goes wrong.