Here is the official dell document on how to reaseat keys:
It is possible for the keycap on the internal keyboard to get stuck or come off in the course of using a portable computer. To put the keycap back on the portable, perform the following steps:
1. Determine how the keycap connects to the portable system.
Underneath the keycap is a rubber plunger and a plastic scissor spring. (see Figure 1 and Figure 2)
Figure 1 - Bottom of Keycap, Scissor, and Plunger on a Portable Internal Keyboard
Figure 2 - Portable Internal Keyboard with the Scissor in the Raised Position
One style of portable keyboard also includes a scissor support bar. (see Figure 3)
Figure 3 - Optional Support on Portable Keyboard
2. Carefully line up the keycap on top of the plastic scissor and press down. (see Figure 4)
Figure 4 - Lining up a Keycap and Pressing Down
NOTE: You might need to wiggle your finger back and forth for the keycap to snap down on the scissor spring. There are two "snaps," one at the top and one at the bottom of the key.
If there is a scissor support bar and it has come loose, clip the lower part of the bar on first under the bottom scissor rail, then snap the upper part over the top scissor rail before lining up the keycap to reattach it. (Figure 5)
Figure 5 - Clipping the Support Bar Under the Bottom Rail