This should never need to be done. Once an asset is consumed, it is gone from the world, and so nothing should need to do anything more with the Consumable Asset (alm_consumable) record. Because of that, there are out-of-box Access Control (ACL) Rules to prevent this happening.
However, mistakes can happen, and it may be necessary to correct the state, or number remaining, to bring it back to life.
2 ACL rules prevent Consumed Consumables being edited:
If you open either of those you will see the Condition that includes 'State' IS NOT 'Consumed'
These specific SYS IDs are in New York, and at least several versions prior to that. Different ACLs might conceivably be involved in future versions. Turning on "Session Debug" for "Debug Security" would allow you to easily see which ACLs are relevant on a consumed consumable form.
Note: This is a temporary procedure, which needs to be reverted afterwards. In normal use, Consumed Consumables need to remain read-only.
The 2 ACL Rule records will now be back as the Out-of-box version, and so won't break anything in your next upgrade. (If you had simply re-entered the condition and saved the ACL again, this change would have remained as your customisation, and been skipped in the next upgrade, meaning you will not get future upgrades to these ACL rules, which is bad because this could break functionality in future versions.)