I have two master pages, "2-col Master" and "4-col Master". I start out with a 4-col page and insert text. InDesign will insert the remaining pages (each with 4-cols) until the text is complete. How do I apply the "2-col Master" to certain pages?
Right click (just about) anywere in the
Pages window → select
Apply Master to Pages… Then select the desired
Master Page and type in the page numbers of pages you want to be applied.
For convenience: if you have some pages already selected in the
Pages window and then select the
Apply Master to Pages… option, InDesign will fill the page numbers of the selected pages for you.
Koiyu's answer is excellent in situations where you want to apply a different master to several pages at once. If it's only one or two, the quick and simple solution is to drag the master you want to apply from the "Master Pages" section of the pages panel and drop it on the thumbnail of the page you want to change.