5v adaptor to 6v 6.4w UPDATED
First of all, to provide 6.4W at 6V, you need to have about 1A at 6V or 1.25A at 5V. You may get away with 1A 5V adapter because devices usually specify slightly more than their maximum power requirement.
Second of all, as long as you get a step-up converter capable of providing at least 2A (I would recommend a 3A converter so that it doesn't run hot at 1A), you should be fine, but it has to be able to run at 5V or even 4.5V at its input.
 You could use any adjustable step-up (or boost) module with an LM2577 or XL6009 (slightly better than LM2577, according to specs).
Here is one as an example:
https://www.aliexpress.com/item/32762092828.html?spm=a2g0o.productlist.0.0.21975c735s1w7H&amp;algo_pvid=5fa4a4d4-f49c-473d-82df-5dd3d10e15c7&amp;algo_expid=5fa4a4d4-f49c-473d-82df-5dd3d10e15c7-1&amp;btsid=0ab50a5715870295770894662e90dc&amp;ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_