Upright moults do happen
Although back-first moulting is common, some tarantulas successfully moult on their side or in a more upright position, especially inside tight retreats.
Do not reposition the spider
Trying to flip or move a moulting tarantula is more dangerous than the unusual position itself. Let the animal control its own posture.
Burrows change what you see
A fossorial species may moult against the wall of a burrow or in a position that looks awkward from outside. The hidden retreat can support the spider in ways the keeper cannot see.
Signs of progress
Look for gradual separation from the old exoskeleton and eventual free movement. Moulting is slow, and long pauses can occur.
When it becomes concerning
Concern rises if the spider is visibly trapped, leaking fluid or unable to free major body sections after prolonged effort.
Aftercare
Once the moult is complete, leave the spider undisturbed and wait for the fangs to darken before feeding. Keep water available nearby.