While the number of votes you have are limited, it’s possible to remove an existing vote to free one up.
- Go to your profile page (e.g. click your icon in the top right corner, then click your username.) Mine is https://forum.inaturalist.org/u/bouteloua/
- Under Activity choose Votes
- Select a lower priority feature request that you’d like to remove your vote from
- Click/Tap Voted and then Remove vote
- Head back to the other feature request that you want to vote for, then vote!