If one did not do Hataras Nedarim on Erev Rosh Hashanah, should it be performed on Erev Yom Kippur?^[1]

Yes, it is to be performed during Aseres Yimei Teshuvah, and hence if by Erev Yom Kippur it has still not been done then it is to be done on Erev Yom Kippur.

[1] Shlah beginning of Tractate Yuma