Hi,
Search online for [https://www.google.com/search?q=tecno+y3s+firmware+download&client=firefox-b-d&ei=2xJJY960MOy12roPo--0yAU&ved=0ahUKEwiejomgnN_6AhXsmlYBHaM3DVkQ4dUDCA4&uact=5&oq=tecno+y3s+firmware+download&gs_lcp=Cgdnd3Mtd2l6EAMyBAgAEB4yBQgAEIYDMgUIABCGAzIFCAAQhgMyBQgAEIYDOgoIABBHENYEELADSgQIQRgASgQIRhgAUJQLWJQLYKEOaAFwAXgAgAHcAYgB3AGSAQMyLTGYAQCgAQHIAQLAAQE&sclient=gws-wiz|Tecno Y3s firmware download].
There's lots of websites that have it along with instructions how to install it.