may depend on the sort of ticket and the exact combination of stations involved.
some 'advance' tickets are very specific, and (for example) a liverpool street to stanstead ticket would not be valid if you tried to get on at tottenham hale, similarly if the ticket is valid on only one train operator's services, and you want to make a similar (or even the same) journey on another train operator's services, then it's not valid.
a standard single / return is usually valid for getting on or off part way through the journey it's for.
but travel to a 'nearby' station on another line is questionable. there are some journeys where a ticket that says it's to A is also valid to B, but it's complicated. the
national fares manual is public domain but meh.
if you have a specific in mind, then you could always go to a staffed ticket office and ask.
alternatively, there is a
fares advice section on rail uk forums, but they will want specific origin / destination of ticket and proposed journey, and if you've already got the ticket, what sort of ticket and so on.
stanstead does not appear to be do-able on oyster or contactless (
a few places outside london are now - some only do contactless not oyster.)