If there are other new changesets in your working branch, by default, if you update (or try to check in your pending changes), you will need to merge your local changes with the head of the branch.
You can potentially create a sub-branch to check in your changes, but if you want to check in your changes in the current branch, you are forced to unify your changes with the head of the branch.
If you want to discard all the new changesets in the working branch, you can delete all the new changesets you want to discard or you can use the subtractive merge operation to remove the group of changesets you want to subtract:
But as I said, I have not updated for a while and I know nobody else has updated anything new.
So there should be no other changesets there.
While I love Plastic, in that regard, the old version control system of Unity (Collaborate) was so much better and straight forward.
Merging the (erroneously listed as) new changesets from the server with my local version would destroy all the work and changes I have done locally and that is not something anyone would want.
If you are using the Version Control plugin inside Unity, you can open the “Incoming Changes” view. If there are some items there, it means your workspace is not pointing to the head of the branch. It means somebody created new changesets in the branch or you switched the workspace to a previous changeset at some point.
You can also open the “Changesets” view to figure out where your local workspace is pointing and the details of the new changesets in the branch.
If you want to submit your changes without updating your workspace and download the new changes, open the “Branches” view and create a new branch. You will be able to submit your pending changes in the new branch.