Marvin JS does not differentiate between SRU, Repeating units, and Link nodes in display or creation.
However, there are restrictions to export a group as a Link node:
there should be one atom in the group
the repeating pattern shouldn't have been set (or it must be the default value)
the number of repetition should be a range starting with 1.
Select the Repeating Group tool, then select the atom you would like to change to a link node with it.
Then the repeating group dialog opens. Leave the repetition pattern on the default value and set the repetition range from the value 1 to the number you would like, then click OK.
After the dialog closes, the link node is established.
Select the atom you would like to group with one of the Selection tools.
Click on the Repeating Group tool.
Then the repeating group dialog opens. Leave the repetition pattern the default value and set the repetition range from the value 1 to the number you would like, then click OK.
After the dialog closes, the repetition unit has been established.
You can change a Link node to a Repeating unit or an SRU by changing the range, or the repeating pattern.
Changing the atom type in this case is not possible.
Right-click on the atom in the brackets, then select Ungroup from the menu.
When you modify the atoms or bonds in the group, it will automatically ungroup.