How can i update pages for OS X El Capitan?

The last available update for Pages on El Capitan was v5.6.2. Apple does not maintain full installers of older Pages products, so you are out of luck on El Capitan.

From Menu : About This Mac panel, locate the mac model string in parenthesis (e.g. Late-2013, etc.). Use that as your determinant in the following hardware requirement links.

If your Mac meets the hardware requirements for High Sierra, or Mojave, then you can perform the upgrade, and then you can get the current version of Pages from the Mac App Store — which requires either of these two operating systems to be installed. This minimum operating system requirement may change in the future.