You should scale the encounters to increase the difficulty for your party.
The ability to scale encounters to your party is very important when you do not intend on following a campaign to the T. The DMG actually clarifies how to do this for your party on page 82. Where it tells you how to gauge your party's difficulty rating by XP values. Within this section of the book, you can adjust the encounters appropriately.
The way to do this by the book is to gauge the combined XP difficulty of your party:
XP Thresholds by Character Level
\begin{array}{l|llll}
\text{Level} & \text{Easy} & \text{Medium} & \text{Hard} & \text{Deadly} \\ \hline
\vdots & \vdots & \vdots & \vdots & \vdots \\
7 & 350 & 750 & 1100 & 1700 \\
\vdots & \vdots & \vdots & \vdots & \vdots \\
\end{array}
If you have seven players who are level 7, the XP Thresholds By Character Level table specifies:
A Hard encounter for that party would have a threshold of 1100XP * 7 players = 7,700 XP.
A Deadly encounter, by this table, would have over 1700XP * 7 players = 11,900 XP.
When creating the encounter, gauge how many creatures it has, and use the XP from their statblocks in the monster manual (or elsewhere) to determine if they meet this threshold. In this case, let's try using ochre jellies as a test creature for this encounter (though a harder encounter would have more variable creatures than just one type).
The Ochre Jelly has CR2 (450XP). The table on p. 82 of the DMG specifies that you do the following to gauge encounters:
- Add up the XP values of all the monsters in the encounter.
- Multiply that XP value to determine its challenge based on the Encounter Multipliers table on the same page (82).
- Compare it to the party's XP challenge value calculated earlier (7,700, hard / 11,900 deadly) to gauge difficulty.
- The highest threshold that that combined XP value surpasses indicates its difficulty.
So normally 7 Ochre Jellies would only be 3150 XP. But with the creature quantity multiplier, 7-10 monsters is 2.5X. Bringing that XP total to 7875. This would constitute a Hard encounter by the reasoning within the DMG.
\begin{array}{ll}
\text{Number of Monsters} & \text{Multiplier} \\ \hline
1 & \times 1\\
2 & \times 1.5\\
3\text{–}6 & \times 2\\
7\text{–}10 & \times 2.5\\
11\text{–}14 & \times 3\\
15+ & \times 4\\
\end{array}
Ultimately, it is your job as a DM to scale up the encounters if the party has improved beyond what the campaign has originally stated. This is a tool provided by WoTC to help DMs determine how to cater to custom campaigns and/or premade campaigns that are adjusted.
I personally find that Deadly level encounters are the only way to give my players a run for their money. Anything below it can be made trivial pretty easily.