By workflow/issuetype you might be referring to workflow schema.
All you have to do to change the workflow schema assigned to a project is open the configuration for that project (cog icon bottom left of the screen) and then click on workflows (left column). You'll see what worflow schema is currently in use and you can choose another one by clicking on change schema (button in the top right of the screen) and selecting the schema from the list presented to you.
To do this, of course, you'll need admin rights and having previously created the schema you want to apply.
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