=(A1/86400)+25569
...and the format of the cell should be date.
##### you probably don't have a real Unix time. Check your
timestamps in https://www.epochconverter.com/. Try to divide your input by 10, 100, 1000 or 10000**A1 with the cell containing the timestamp ;-pUnix system represent a point in time as a number. Specifically the number of seconds* since a zero-time called the Unix epoch which is 1/1/1970 00:00 UTC/GMT. This number of seconds is called "Unix timestamp" or "Unix time" or "POSIX time" or just "timestamp" and sometimes (confusingly) "Unix epoch".
In the case of Excel they chose a different zero-time and step (because who wouldn't like variety in technical details?). So Excel counts days since 24 hours before 1/1/0000 UTC/GMT. So 25569 corresponds to 1/1/1970 00:00 UTC/GMT and 25570 to 2/1/1970 00:00.
Now please note that we have 86400 seconds per day (24 hours x60 minutes each x60 seconds) and you can understand what this formula does: A1/86400 converts seconds to days and +25569 adjusts for the offset between what is time-zero for Unix and what is time-zero for Excel.
By the way DATE(1970,1,1) will helpfully return 25569 for you in case you forget all this so a more "self-documenting" way to write our formula is:
=A1/(24*60*60) + DATE(1970,1,1)
P.S.: All these were already present in other answers and comments just not laid out as I like them and I don't feel it's OK to edit the hell out of another answer.
*: that's almost correct because you should not count leap seconds
**: E.g. in the case of this question the number was number of milliseconds since the the Unix epoch.