Unable to download e-books purchased several years ago

I  have a new mobile phone device and laptop PC. I am unable to download my e-books purchased several years ago. (Using Adobe Digital Editions ID)

According to the website  - see below, I have to reset my activations, but who do I contact to  assist me?

You can activate up to six computers and devices. If you reach the limit, contact Customer Service to reset your activations.

Please assist me.

Thanks

Chris