Hi and welcome to the forum!
The “Restart” feature is explained in the FAQ area but care is needed before deciding to use it
If you click on “Restart” (either on the course homepage or on a ‘level’ page within a course) it will wipe your learning to date for either the whole course or the whole level, and you will then be able to go through the planting and reviewing cycle from scratch. Your “Words learned” total will be reduced but you will not lose your points earned to date. So, if you decide to go down this road, it might be best to try it first with a single level of a course.
An alternative that you could consider would be to take some of the ‘user-created’ courses (the quality of these varies but there are some very good ones). You can find these in the course library in the web version of Memrise and, once you have started a course on the web, it will sync to any versions of the Memrise app that you have so that you can continue learning them there. Sorry if you already know all this!
Hope it helps.