To replace it simply remove the MX84 from the network using the dashboard and then add the MX250.
Note that the network will retain the existing software imposed WAN speed restrictions of the MX84, so you'll need to go into "Security & SDWAN/SDWAN and Traffic Shapping" and increase the speed to a minimum of their Internet circuit size, or the maximum.
Also note the MX250 only has SFP+ WAN interfaces - so if you want to plug in a copper Gigabit Internet circuit you'll need to get yourself a Gigabit SFP.