If you are trying to do this from a formula into another cell, and not in situ (For which, use "Text to Columns" as per Jeeped's answer), you can either combine DATE and MID, or use REPLACE and convert with +0 or DATEVALUE:
=DATE(MID(A1,1,4), MID(A1,5,2), MID(A1,7,2))
OR
=REPLACE(REPLACE(A1,7,0,"-"),5,0,"-")+0
OR
=DATEVALUE(REPLACE(REPLACE(A1,7,0,"-"),5,0,"-"))
(Where A1 is the date to convert)
The first formula just cuts the number up into 2018 01 01 and uses those as Year, Month and Day. The second 2 work by first Inserting (i.e. REPLACE 0 characters) a hyphen at position 7 ("201801-01") and then at position 5 ("2018-01-01") and converting the string back to a number/date.