We do not have support for updating Uccelerate to the latest version since updates would delete any possible modifications made to the theme. You can however do a merge of the files from the latest package into your own solution to the newest version. There might however be breaking changes in between versions, so do this at your own risk.
Checkout stuck on loading / 500 error from the checkout API
This is most likely due to Uccelerate not being able to find a shipping or payment method from Ucommerce or not having the right security level set. Make sure everything is set up correctly in Ucommerce under settings.