Buy the correct part number from the Gardena website or from a retailer. Completely empty the RollUp’s hose. Open the housing (usually a few Torx screws or clips). Remove the old hose guide (it is often clipped or screwed in place). Fit the new hose guide in the same position. Reassemble the housing.
Most RollUp models (S, M, L, XL) offer fairly easy access to this part, as it is a component that wears out or breaks over time (especially if the hose rubs against it a lot).