These instructions have been provided by the Legacy of the Dragonborn mod page, and while written for updates of Legacy, the step by step instructions below still hold true for SUT.

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Save game cleaning can help fix many problems you game may be experiencing, however it can also break your save game entirely if you don't do it correctly. You should never do a full across the board clean of every entry in your save game, that will most likely spell disaster, but if you carefully and precisely remove the correct entries, your game will survive and run faster and better for it.

Here are the instructions needed to run SURGICAL cleanings of your save game for
Legacy content that must be cleaned. I try and keep calls for this to a
minimum, but now and then a bug is found in the Legacy scripts that must
be cleaned in this manner. In an update that requires this i will list
the specific script name that should be purged from your save in the
update itself. Just refer back to this tutorial for the steps on
removing it.

Before we get started I feel a little explanation on WHY this must be done is in order. If you understand already, have read
this before or simply don't care to know how your car engine works as
long as it works, feel free to skip ahead.

When Skyrim loads up for the first time it loads EVERY SINGLE SCRIPT in the game and when you
save your game all properties of that script are saved to your save
game. Congratulation, they are now BAKED into your save and can never be
changed or removed under normal circumstances. This is normal. When a
mod updates and makes a change to any script, the author can add new
properties, add and remove functions from the script but CANNOT remove
any properties or change what the properties are pointing to in the
game. EX: I have the Rueful axe display set to accept the rueful axe and
all other 5 display options are filled with a dummy object because they
aren't needed (but chillrend would need them for instance because there
are 6 leveled options for it). I decided later to add a replica option
for the Rueful axe but I cannot simply change one of the dummy
properties to point to the new replica option. It will work for NEW
games but the property is already baked into existing saves.

So enter the save game script cleaner. By removing all instances of te
DBMweapondisplay script (the script which handles all weapon displays)
it will make Skyrim have to re-fill the properties from the ESP WHICH if
updated to a new version of legacy will fill one of said properties
with the new replica option for the Rueful axe. Problem solved.

The second reason that I may require a script clean is when a scripted
object is removed. EX: When I overhauled the safe house I removed a
light source for the sun which was scripted to shut itself on and off at
certain times. When the object is removed, the script still stays in
your save file. Using the save game cleaner program will allow you to
remove scripts from your save game that are attached to deleted objects.

Failing to do either of these cleaning procedures will result in Ppayrus errors and possible CTD down the line.

Now that that's all out of the way, here are the steps...

Cleaning your save game using Save Game Script Cleaner

Please note. The cleaner above is required for this. There are other utilities
which will actually break some displays in the museum because they
THINK that the havoc is broken and will delete them which is simply
wrong and stupid for it to be doing in the first place. ONLY use Save
Game Script Cleaner for this cleaning of legacy.

1- Download the Save Game Script Cleaner (no brainer)
2- Open said program
3- Load your current save game file

Cleaning specific script instances

1- Enter the script name in the SCRIPT search box
2- Highlight said script name under INSTANCES
3- Click DELETE
4- Highlight said script name under SCRIPTS
5- Click DELETE

Cleaning deleted scripted objects

1- Click ADVANCED
2- Click Remove Scripts Attached to Nonexistent Created Forms

After cleaning is done, save your new save game. A backup will be created of your old save file.

THAT'S IT!

As long as you have not touched any other options, your game should
continue to run smoothly. The reason that save game script cleaning in
general CAN break your game is if you go on a spree and clean
everything. By removing an interconnected script which refers to
elements of another script it can cause wide spread failures. Scripts
that I have you clean for Legacy will NOT be an interconnected script
but rather a stand alone script, so removal of said script is 100% safe
as the game will simply re-apply it with the updated content when you
load again.