Welcome to the QuickBooks Community, @ANAK11.

Thank you for providing details about your concern. Let me share you the steps in making multiple items inactive:

1. Go to the Lists tab, select Add/Edit Multiple Lists Entries. 
2. Let's add a new column for the Inactive option. Click Customize Columns.
3. From the Available Columns field, choose Inactive.
4. Hit Add> to transfer the Inactive option under Chosen Columns.
5. If you want to move the Inactive column, just click Move up and once done, tick OK. 
6. After that, let's proceed to the process. From the List drop-down, select the type of Inventory.
7. Pick the specified category of your inventory on the View drop-down. 
8. To make items inactive, put a checkmark under the Inactive column, then Save Changes when finished. 

Then about deleting the wrong quantities on your Inactive Item, you can adjust your Quantity on Hand. This process will help to keep your inventory status in QuickBooks accurate. Follow the steps below:

1. Go to the Inventory tab, choose Adjust Quantity/Value on Hand.
2. Pick the necessary information for the Adjustment Type, Date, and Account.
3. If you don't have an Adjustment Account, let's first set that up.
4. From the Adjustment Account drop-down, click Add New.
5. Account Type is Cost of Goods Sold, then enter Inventory Adjustment for the Account Name.
6. Select Save & Close once done. 
7. To change the Quantity on Hand. From the New Quantity column, enter 0 (zero) to remove the wrong quantity.
8. When finished, hit Save & Close. 

Also, for the App, you can go to QuickBooks Apps Store and from the search bar enter the keyword "Importer". This will help you to find supported Third-Party Application that suits best depends on your QuickBooks version.  

Additionally, to transfer your Items from a site to another, you can refer to this article for more details: Multiple Inventory Sites. This article provides detailed steps in moving your inventory to other locations.

Moreover, in case you want to make you Items active again, I'll guide you how:

1. Go to the Lists tab.
2. Select Item List.
3. Tick the box beside Include Inactive option to show all the Inactive Items.
4. You can see X marks for the inactive items.
5. Click the Items with X signs and that makes them Active again. 

Let me know if you have any other concerns. I’m always ready to answer them for you. Have a good one.