1. Import Your Video Footage
Launch Wondershare Filmii, make sure that you are signed in to your account, ensure that the Create option is selected in the left pane, and click Start from Scratch from the right. When the new window opens up, click Import File from the upper-left window, and select and import the source video footage from your PC to the program. Now, hover the mouse over the imported clip, and click the + (Add) button that appears at the bottom-right corner of the thumbnail to add the video to the Timeline.
2. Adjust the Speed of Your Video Clip
Position the Skimmer (Playhead) on the frame in the Timeline you want to start the modified speed from and click the Split button from above the Timeline. Likewise, position the Playhead on the frame you want to modify the speed to, and click the Split button again. Now, select the segment of the video you isolated, and click the Change Speed and Duration button from the right toolbar above the Timeline. On the Custom Speed box, enter your preferred speed value in the Speed input field (e.g., to double the speed, enter 2, or to slow down, type 0.5 or something similar). Optionally, you can also drag the Speed slider to the right or left to increase or decrease the playback speed of the video respectively. Click OK when you are done.
Note: You can even turn on the Play video in the reverse switch to reverse the playback.
3. Export Your Video Clip
Click the Play button in the Preview window (present at the upper-right section) to see if the speed has been adjusted as expected. Once everything looks fine, click Export from the top of the interface, and follow the on-screen instructions from there to export the modified video in your preferred format, or for your favorite social media or streaming platform.