While holding, connect the phone to your PC via micro-USB and re-insert the battery.
For the Samsung GT-C3520, the flash file typically comes with a extension. This is a proprietary Samsung format used for older Broadcom-based chipsets (BCM2133). Unlike modern Android phones that use .tar or .md5 files, feature phones like the C3520 require a complete memory dump or a PAC file compiled via Samsung’s Singapore Tool. samsung gtc3520 flash file and tool
You're looking for information on the Samsung GT-C3520 flash file and tool. Here's what I could gather: While holding, connect the phone to your PC