Lock the 2nd page in word, while making chages in the 1st page

This is possible, though as Paul mentions, not foolproof:

1. In Word, select the content on Page 1 that you want to be editable.
2. Click on the Review tab, then select Restrict Editing.
3. Under Editing Restrictions, check Allow only this type of editing in the document. Leave the dropdown below set to the default No changes (read only).
4. Under Exceptions, check Everyone.
5. Click on Yes, Start Enforcing Protection. Save the file.
6. Test whether the document operates the way you want. You should be able to edit in the areas you had selected and nowhere else.

You could insert a manual page break at the bottom of page 1, but what do you expect to happen if, post-editing, the material on what is now the first page flows over to the next page, as your existing page 2 will now become page 3...