How to change SCCM reserve disk space for a DP

1) Run SQL query:

select SCR.ID, SCR.Name,SC.NalPath, SCR.Value3 from sc_sysresuse_property SCR join sc_sysresuse SC on SCR.SysResUseID = SC.ID where SCR.name = 'MinFreeSpace' and SC.NALPath like '%%' and SC.RoleTypeID = 3

get SCR_ID

2) Use the returned ID in step 1). Run the below query to update the 'MinFreeSpace' value.
update sc_sysresuse_property set value3 = 'set desired value in MB' where name = 'MinFreeSpace' and ID = ''
Example:
update sc_sysresuse_property set value3 =10240  where name = 'MinFreeSpace' and ID = xxxxxxxxxxxxx

3) Change registry value HKLM\software\Microsoft\SMS\DPreserveddiskspace to match the  number you used on step 2, eg: 10240. 

You don't have to reboot the DP.

Restart the SMS_SITE_Component_Manager service on SCCM site server.