If you have already launched your study there are a number of things that can't be edited. You can find out more about what is locked after getting participants here.
The reason for this rule is to ensure that Participant #1 and Participant #100 have the exact same experience. This ensures that you receive robust results without inconsistencies. If the study changed halfway through fieldwork, those results would be incorrect and unreliable.
If you still require edits to be made, you will need to duplicate your study which will generate a 'copy of' the study you want to edit. You can make any changes required on this 'copy of' version and relaunch from there.
How to duplicate a study
To duplicate your study, head to your Studies page. Next, click the white Actions button next to the title of the chosen study, then click Duplicate.
Please note this new study will collect a seperate pool of data that cannot be merged with the data collected on your original study.