I jumped the same hurdles when I increased my contribution a year ago. It should be easy - but it’s not. Multiple emails were involved, and the top & bottom is that I had to cancel my membership and start a new one with a higher contribution level.
Around the same time, I emailed Dale with a list of ideas to increase membership, along with increasing contributions from existing members. #1 on the list was an ‘Increase Contribution’ button on the website.
The button never appeared, and I never got a reply.