If you have created activities with detailed descriptions and you do not want to have to write everything again, you can copy activites from one project to another.
In order to do this you can go to the project's main page on the project you wish to copy activities to, and then click on the cogwheel icon on the top right:
You will then get this window, asking you to choose which project you wish to copy activities from:
You will then be able to copy the activities along with the tasks. You can also choose set the activities either as "not started" or "in progress".
When you click "save" you should be done and all the activities you have chosen will have been copied.
This can be done simply by moving the hours from the "hours overview" report in Moment.
Go to a project, open the hours overview report. In there. mark all the hours that you want to move, click on the column "activity" and select a new activity:
A small prompt might appear, regarding task assignments. You can disregard that for now. Once you choose the relevant activity click "okay" and the hours should have been moved.
Deleting activities is in general not possible unless you have removed all the hours from it (by moving them as described in the section above) and as long as there aren't any invoices that have any references to said activity. In general we recommend setting activities as "finished" and renaming them.