How do I modify the title of a paper I submitted to ArXiv?
Short answer: I believe the only way is to submit a revision.

Long answer: As per this page:

It is not possible to make changes without generating a new version

Why? To allow authors to make changes to the comments and other metadata fields of publicly announced articles without generating a new version, and hence date stamp, would effectively mean that all changes would be backdated to the time the last version was submitted. This is open to abuse and thus not allowed. 

The exceptions to this rule are the addition of journal reference and DOI information. The journal reference field may contain only full publication information for the article; the DOI field may contain only a DOI.



My suggestion: Although this replacement process would not be time consuming, it would be quite redundant and generate a new version of the paper. Since that would anyway happen whenever you revise your paper (say, after you incorporate the comments of referees), I would suggest -- under the assumption that the typo does not look glaringly bad -- to wait until you have a next version ready.