You want to change the priority of a transport rule.
You are able to change the priority of transport rules using the Exchange Admin Center (EAC). The user experience is awful. A faster way is to use Powershell.
If you want to change the priority from XX to YY:
Connect-ExchangeOnline
# complete authentication flow
Get-Transportrule | ?{$_.Priority -eq XX} | Set-Transportrule -Priority YY
All rules with a priority greater than or equal to the new priority of your rule will be increased by one.