You should be able to accomplish editing SPM test thresholds through Attribute Editor.
For example, let's say you would like to change the Critical value for a group of ICMP tests
1. use Locator search to pull up all SPM tests
2. select the group of SPM tests based on column Test Type
3. right-click > Attribute Editor
4. go into User Defined and choose "Add"
5. look up "latency" - you can filter for the "Latency CritSetValue" attribute 0x456009d
6. you can then edit the value (for example, put in 1000 ms for Critical threshold)
7. all selected tests should then reflect the same change
As another example, lets say you want to set the Packet Loss Critical threshold on a group of tests found in the Locator search.
You can run the same steps above but instead choose the PacketLoss CriticalSetValue 0x45600a0 attribute for step 4.
Conversely, you can also adjust the major and minor threshold values for these tests so the default values do not conflict with your new Critical values.