You can instruct Xert to go through all your activities, re-extract fitness signatures, look for fitness breakthroughs, and recalculate MPA, strain, XSS and XEP for each activity.  This is like an account "reset" as it throws out all previous calculations and recalculates them all over.  Use this to correct errors that have manifested in all your data.  This can happen if an activity has invalid data and has had this applied to your entire progression data.
To have Xert do a progression recalculation, go to your Account Settings / Profile tab:
Click the Recalculate Progression button to have the system go through an recalculate all your signatures.  The system will calculate your starting fitness signature for you based on the Starting Training Load estimate.
Pro tip
If you're looking to fine tune your progression history, you can adjust your Fitness Signature which will force the progression recalculation to start with this signature.  Ideally you would also change your Starting Training Load to correspond with this Fitness Signature for best results.
Note that during the recalculation process, any activities that have saved/locked fitness signatures will be unlocked and updated.