software update instructions for Asha 501 can be found from here:
By default your phone is set to automatically check for updates every 8 days and you will be notified when an update is available for download. If you want, it's possible to force the update check immediately by switching off and back on "Check via mobile data" setting from the “Phone update” menu. The size of the update is around 8 MB, so it's recommended to use Wi-Fi for the download.
Check out also the attached .pdf document and/or this video:
Nokia Asha 501 OTA software update onepager v1.0.pdf
Nokia Asha 501 OTA software update onepager v1.1.pdf