First of all please download a new Authorize.Net SIM files attached to this topic and upload to your Viart payments subfolder.
 
Then you will need to generate a new Signature Key in your Authorize.net account.
1. Log into the Merchant Interface at https://account.authorize.net.
2. Click Account from the main toolbar.
3. Click Settings in the main left-side menu.
4. Click API Credentials & Keys.
5. Enter your Secret Answer.
6. Select New Signature Key.
To disable the old Signature Key, click the check box labeled Disable Old Signature Key Immediately.
If the Disable Old Signature Key check box is not selected, the old Signature Key will automatically expire in 24 hours.
7. Click Submit to continue. Your new Signature Key is displayed.
8. Save a new Signature Key to clipboard.
 
Now you will need to set a new Signature Key and update Authorize.SIM settings in your Viart Shop
1. Log into administration backend of your site.
2. Go to 'Settings > Orders > Payment Systems'
3. Click 'Edit System' link next to Authorize.net SIM
4. Paste your new Signature Key for Parameter Source field of x_secret parameter.
5. Paste './payments/authorize_sim_checkout.php' value without quotes in your Payment URL field above replacing old value 'https://secure.authorize.net/gateway/transact.dll'
6. Click 'Update' button at the bottom of the form to save your changes.
 
Also you will need to update validation script in your Final Checkout Page settings
1. From 'Settings > Orders > Payment Systems' page click 'Final Checkout Page' link next to Authorize.net SIM.
2. Paste './payments/authorize_sim_validate.php' value without quotes in your Validation Script field replacing old value './payments/authorize_check.php'
3. Click 'Update' button at the bottom of the form to save your changes.
 
When you've done you can try to submit a small order to validate that your recent changes works correctly.