Hi,
Do you mean that the battery housing back cover is broken?
Here’s a link to the ifixit [guide|32636] guide.
It is useful as it shows how to remove the battery housing back cover so that it can be replaced. (you only have to go as far as Step.4 in the guide)
Here’s a link to [https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2499334.m570.l1313.TR1.TRC0.A0.H1.XNokia+Lumia+635+back+cover.TRS0&_nkw=Nokia+Lumia+635+back+cover&_sacat=43304|suppliers] of the replacement part.