hi,to update play store try this methods . clear cache in settings>storage>cached data>clear cache.
[1] open play store tap on menu [three line on top left]and scroll down for settings tap on this ,scroll down to build version
tap on this ,play store update automatically,let it finished.reboot/restart device
[2] go to settings>storage>apps scroll down for google play store tap here and tap on uninstall updates,play store update automatically,let it finished.reboot/restart device.
[3]go to settings>> security >unknown sources next to this "check" in the box/enable the switch ,return settings >storage>apps> scroll down for play store tap on this and tap to uninstall updates.
finishing this open chrome browser type in google search box "
www.apkmirror.com/apk/google-inc/
google-play-store/ " hit, see in next page latest version of play store, tap on this and see play store button /download button or scroll down to download button
tap on this, a warning pop-up comes,tap on "ok" button .see play store downloading . finally allow to install.after finished process go to settings>>>>>>unknown sources > "uncheck" in the box.
Reboot /restart device.