You can change it on the filer by logging into priv set diag mode..First verify your thresholds before changing it.In most cases, default makes sense but again it varies case by case and your environment.
Let me explain the risk before providing the command.
First of all you need to understand the data size being written to your volumes. If you have a 1TB volume with 98% threshold, you have 20GB left.If you change to smaller percentage, it will start consuming the free space quicker and you would run out of space on the volume.
Also consider, what is the overcommit on your aggregate. If you already overcommited, Use IScsi LUNs use caution.
Think of enough warnings...Here is how you change the settings.
To print the existing values, Type
priv set diag;print flag wafl_reclaim_threshold
To set new values, BASED UPON YOUR ENVIRONMENT(see my writing in bold make sure you do some ground work).
priv set diag;set flag wafl_reclaim_threshold_t (tiny volumes <20GB)
priv set diag;set flag wafl_reclaim_threshold_s (small volumes >20GB & <100GB)
priv set diag;set flag wafl_reclaim_threshold_m (medium volumes >100GB & <500GB)
priv set diag;set flag wafl_reclaim_threshold_l (large volumes >500GB <1TB)
priv set diag;set flag wafl_reclaim_threshold_xl (tiny volumes >1TB)