You simply have to have the contract you wish to deploy in the active tab.

Here is a full rundown in case:

 1.  **download back the same contracts to Remix:**

[![enter image description here][1]][1]

2. **open the file you wish to deploy in a new tab by clicking on it and stay on it:**

[![enter image description here][2]][2]

3. **make sure to compile them (I generally tick the 'autocompile' checkbox):**

[![enter image description here][3]][3]

4. **select the contract you wish to deploy in the dropdown menu:**

[![enter image description here][4]][4]

5. **use the `at address` to bring up the already deployed instance of your contract:**

[![enter image description here][5]][5]

And there you go !


  [1]: https://i.sstatic.net/ocTe5XA4.png
  [2]: https://i.sstatic.net/Hx5FEQOy.png
  [3]: https://i.sstatic.net/YRHog3x7.png
  [4]: https://i.sstatic.net/XICKRR1c.png
  [5]: https://i.sstatic.net/AJAQB9u8.png