How do I reset my routers?

Kestrel from over in Malware sent me here for help on how to reset my routers. I'm having some sort of persistent browser pop up issues that have defied detection across all five machines on my network.

I've got a dsl modem/router and an additional wi-fi router in the house, and another wi-fi router in the office 200' away.

I'm only minimally self-educated on network stuff, so please move slow and assume I know nothing.

thanks
Off White
Tenino, WA