How do you perform a software reflash?

[AC Question]

I called technical support twice (once in Nov 2015 after activating phone) to trouble shoot problem with phone. Every since phone was activated I been having trouble w/ my certified pre-owned "New" Galaxy S5. Message "Unfortunately, com.sec.imsservice has stopped" causing phone to work improperly or not at all. 2nd time worst – left my phone “downloading…” (no status of % or indication that it is moving and not frozen). “Do Not Turn Off Target!!”