There are two methods to assign Issues to sprint or Release.

By edit:

There are two ways to Edit Issue,

  1. Click on Issue ID
  2. Issue Menu > Edit

In Right side panel you will find options to Select Project, Sprint, Release. Select your Project, Sprint and Release from dropdown and click on Save.

Issue-Assign-to-Sprint-or-Release Planning board

Go to Planning Board.

  1. Select Issue backlog From Dropdown under Issues > Select projects checkbox to see Issues in Issue backlog column.
  2. Select Sprint/Release From Dropdown Under Issues > Select project and sprint/Release from sprint/release column.
Note: You can Open multiple backlogs in planning board)

Now you can easily move your Issues simply drag n drop.